A lot of actors can play Americans for most of their career, and people can often be surprised where they're actually from. Hollywood in particular is full of Australians faking accents. There are plenty of Brits too, but the English accent is often considered more marketable than the Australian onenote Daniel Radcliffe started auditioning to play American characters after Harry Potter and was told he was more marketable with his own accent. - so in general if a Brit is famous in America for a role in their own nationality (Daniel Radcliffe, Emma Watson, Keira Knightley, Jude Law etc.) they're less likely to be called on to play Fake Americans. Emily Blunt is an example of an actress who played more Americans early in her career, but her bigger successes (Edge of Tomorrow, The Devil Wears Prada, Into the Woods, The Adjustment Bureau) all have her playing her own nationality. Foreigners who aren't as lucky include:
Naomi Watts is British born but raised in Australia, and has played Americans for most of her career. The only films where she has played Australians are Mulholland Dr. and Tank Girl.
Hugh Jackman is another Aussie who has played Americans for 90% of his roles. Even though his Star-Making Role in the X-Men Film Series is supposed to be Canadian. He jokes that when he played an Aussie in Chappie he had to look up what the Australian slang words in the script actually meant.
Charlize Theron is South African but has always played American characters (well, nearly always). This is partly because she learned English when she was in America, so she speaks with the accent naturally.
Embeth Davidtz is the opposite; she's an American raised in South Africa, so she fakes the accent in most of her work (save for a couple of Fake Brit roles as well). The film Junebug is one of the rare times she gets to use her real accent.
Anna Paquin is from New Zealand but has been working in America since her teenage years, starting with Rogue in the X-Men Film Series. Her accent has mostly faded now. But hilariously her most famous roles are characters from the Deep South - the aforementioned Rogue and Sookie Stackhouse on True Blood.
Rose Byrne can do a flawless American accent and thus rarely finds herself playing Australian characters. She played her first American in Wicker Park and has alternated between that and Fake Brits for most of her career.
Diane Kruger claims she tried very hard to lose her German accent in order to be able to play American characters. She hasn't quite gotten rid of it, as she still has a hint of an accent. Ironically she had to exaggerate her voice to play the German Bridget von Hammersmark in Inglorious Basterds.
Alexa Davalos is French but has an American-sounding voice, so she only has to change the occasional pronunciation.
Ellen Page is Canadian but has played American characters. Inception might be an aversion, as she's a student in Paris whose nationality isn't stated.
Dominic West hasn't had a role using his own accent outside of the UK. Chicago, The Forgotten, Punisher: War Zone and The Wire all have him using the accent. He describes fans being "a little disappointed" when they find out he's British.
Idris Elba early on in his career went into auditions using a Fake American accent, convinced casting directors would turn him away if he was British. With the success of Thor and Luther using his own accent, this naturally doesn't happen anymore.
Jim Carrey, who is Canadian, in far too many roles to list.
Brendan Fraser (born in Indiana, but the son of Canadians who even attended school in Toronto) also. Amusingly in The Mummy (1999) some of the British characters insult the Americans, before apologising to Fraser's. He responds that he's not offended, possibly as a Lampshade Hanging to the audience.
Catherine Zeta-Jones is another Welsh actor who only seems to play Americans. In fact, 2016's Dad's Army was the first time she had played a British character in years. What's more is that her Star-Making Role was The Mask of Zorro and people assumed she was Spanish for a while. It's telling that she was called on to play a lot of Spicy Latina characters in Hollywood - but has no Spanish ancestry herself. She lampshaded this on The Graham Norton Show:
Graham: Catherine Zeta Jones, amazingly enough from Wales.
Saoirse Ronan was born in The Bronx but moved to Ireland with her Irish Parents as a toddler. She pulled off the American accent flawlessly in her first role at age twelve - in I Could Never Be Your Woman. If her Star Making Role hadn't been as a Fake Brit in Atonement, people would have probably just assumed she was reared in America, with mostly playing Americans (barring The Way Back, Hanna and The Grand Budapest Hotel). The film Brooklyn was the first time she had played an Irish character - a full twelve years after her professional debut.
Claire Forlani is British but started her acting career in America, with predictable results. Even in Meet Joe Black where her father is played by the British Anthony Hopkins, she's still a Fake American. Her first notable role using her own accent was in the series Camelot - twenty years after her professional debut. She's an example of starting out not being very good at the accent (see Mallrats) but improving a lot over the course of her career.
Emily Browning (Australian) has been working in America since she was a child, and has been able to pull off the accent flawlessly. Many viewers of A Series of Unfortunate Events were surprised to discover her real nationality.
Alfred Molina, who plays Doctor Octopus in Spider-Man 2, is from London. When he says the line, "I should've known Osborn wouldn't have the spine to finish you!," he slips up a bit.
Rosemary Harris, who plays Aunt May in the films, is also English.
Daniel Gillies, who play John Jameson in the second film, is Canadian-born and grew up in New Zealand.
The Amazing Spider-Man: The new Spider-Man, Andrew Garfield, was born in Los Angeles to a British mother and an American father, but has spent much of his life in the UK - and so has to fake the accent.
Anthony LaPaglia is in a film called Empire Records, playing a very American small-town record store owner. Although he's all but lost his Aussie accent as a price. Just listen to him here, and compare him to everyone else.
Patrick Stewart (English) and James McAvoy (Scottish) as Professor X, who is confirmed to be half-American in X-Men: Apocalypse when he mentions his grandfather planting a tree on the Westchester estate. Both of them portray the character with English accents, making this a marginal example.
Rose Byrne (Australian) as Moira MacTaggert, where the Scottish scientist of the comics has been adapted into an American CIA agent.
X-Men: Days of Future Past has two fictional examples (French Omar Sy as Bishop, Chinese Fan Bingbing as Blink—then again, both characters are children of immigrants) and a real person (Richard Nixon is Canadian Mark Camacho).
Bob Hoskins' portrayal of Eddie Valiant in Who Framed Roger Rabbit. Hoskins' natural accent is a thick Cockney, not unlike Michael Caine's. Here, he does a fairly convincing hard-boiled New York accent, though the film takes place in Los Angeles. Bob Hoskins plays Mario in Super Mario Bros. with a New York accent. It's one of the few things that worked there. He also attempts something that sounds like an American accent in his self-directed film Rainbow - although he may be going for Ooh, Me Accent's Slipping to imply that Frank has traveled a lot.
Her portrayal of American debutante Rose Dewitt Bukater in Titanic (1997). Kate admitted being "appalled" by her accent when she watched the movie 15 years later for its 3-D release and wishes she could redo it with all her years of experience she's had since then.
Cillian Murphy in Red Eye. Wes Craven was initially wary about casting him as Rippner, since Rippner was explicitly written as American. When Murphy walked into the audition, he actually fooled the casting agent with his accent. (The film also has Canadian Rachel McAdams and Scottish Brian Cox as Americans.)
Gary Oldman has practically made a career out of playing fake Americans: he puts on a Southern accent for The Fifth Element, a Texan accent for The Book of Eli, a New York accent in The Professional; he even played Lee Harvey Oswald in JFK. Infamously, for a period he went into therapy due to losing his actual accent.
In Die Hard, director John McTiernan said he decided to extend the scene where Englishman Alan Rickman's character of Hans Gruber pretends to be a hostage in order to show it off. YMMV as to whether you believe this.
While with the Grubers, in Die Hard with a Vengeance, when Simon shows up at the blast scene and poses as a city engineer, Jeremy Irons puts on a very fake Texas drawl - which works!
Peter Sellers (English) as President Merkin Muffley in Dr. Strangelove. He affects a sort of flat Midwestern accent that is as "generic" as American accents get, particularly since Muffley was partially based on Illinois Governor and presidential candidate Adlai Stevenson. (He also plays a fake German and an actual Brit in the same film). Sellers also played Americans in Lolita (and that character masquerades as a German at one point!) The World of Henry Orient, I Love You, Alice B. Toklas!, Where Does It Hurt?, and Being There. (In the last case, it's an accent whose region can't be identified by the CIA or FBI. Sellers created it by applying an American accent to an imitation of (British) Stan Laurel's voice.) He's one of the more convincing fake Americans when he's "on".
Clive Owen who is British, puts on a not-too-convincing American drawl to play Dwight in Sin City.
In Phone Booth, you have Australian Radha Mitchell as the wife; see also Silent Hill, in which she played the main role (and Sean Bean, below, played her husband, both with American accents).
Sean Bean pulls off a very frightening generic American accent as the main character in the 2007 remake of The Hitcher.
Red Leader in Star Wars Episode IV has a quite convincing "mountain twang" pilot accent (à la Chuck Yeager, Tom Wolfe must have smiled); despite being played by Brit Drewe Henley. Most of the British extras were dubbed over by American actors. Shelagh Fraser redubbed her own voice as Aunt Beru to sound more American, though different takes are heard depending on which sound mix of the film you're listening to.
The American-accented Stormtrooper-turned-Resistance fighter Finn in The Force Awakens is played by British actor John Boyega. According to the man himself, they tested him out using his natural voice, before JJ Abrams quickly told him to do an American one instead. His co-star Daisy Ridley gets to keep her English accent.
Australian actor Hugo Weaving puts on a slightly exaggerated, sneering Midwestern accent when playing the role of Agent Smith. Word of God is that he was trying to imitate director Larry (now Lana) Wachowski.
More convincingly, from Enter The Matrix and a brief bit in The Matrix Revolutions, is Sparks, the operator of the hovership Logos.
On the other hand, Weaving's voice for Megatron in the new Transformers movies does a good job of disguising his accent under a growling tone of voice that well fits this particular incarnation of the character.
Mel Gibson. While he was born in the US, early in his career you can tell he had to make an effort to cover up the Aussie in his speech. You can definitely hear his American accent slip a couple of times in the first two Lethal Weapon films. These days, when he gives interviews, you would be convinced he never left the US since his birth.
No Country for Old Men is chock full of convincing Texan accent and dialect from non-Texan actors (although several actors, like Woody Harrelson and Tommy Lee Jones, were Texan)... but perhaps the most surprising is that of Kelly Macdonald, who's Scottish. The other big one is Javier Bardem, who is Spanish. In fact the voice he used for Anton Chigurh came about by accident when he tried to tone down the accent during a run-through of the script.
Robert Shaw in General Custer of the West. He does a pretty good job, but his accent falls through a couple times.
Max Carrigan, the incorrigible draftee and brother of the female lead in Across the Universe, was played by British actor Joe Anderson with a sardonic, nasal, and reasonably convincing American accent. Even more impressive is that he does a great job of singing in the fake American accent.
The Austrian Charles Rooner (born Ernst Robitschek) made a career out of playing these ones in old Mexican films.
Parodied in Tropic Thunder, which casts an American as an Australian cast as an (African) American. (Originally, the character was supposed to be "truly" Irish, but Robert Downey, Jr.. found it easier to improvise with an Aussie accent.)
James McAvoy as the "hero" in Wanted. He's Scottish, but usually has an I Am Very British voice in most of his roles. General consensus seems to be that his natural accent slipped through a lot.
Michael Caine had to play a Texan in Secondhand Lions. And he did a terrible job, too. In promotional interviews Caine said the secret to the Texas drawl is "to let the syllables lean on each other." He also pulled off lousy American Accents in Bullseye! (in one of his two roles in that film) and The Cider House Rules. And he won an Oscar for the latter, yet! Compare to Charlize Theron's far more convincing tones in the same movie (and, in fact, every other film the South African Miss Theron has made – she has yet to play her own nationality on screen).
Christopher Lee did a nice flat Californian accent in the movie Serial – and floating around out there is a recording of him singing Ghost Riders in the Sky with a Southern drawl.
Averted in Shortbus, where the director tried to get Sook-Yin Lee to not sound so Canadian, and finally gave up and wrote into the script an explanation that her character was Chinese-Canadian.
Jude Law's ridiculously bad American accent in I Heart Huckabees. It's painful. He repeats his performance in All The King's Men. He also did a barely passable American accent in eXistenZ...however, this is justified seeing as how his American-accented character was just an avatar for a character of his own British nationality.
By that same token, the villain "Red" from Hancock puts on a truly terrible fake... something accent; he seems to have a hard time settling on which one he wants to use.
Before House, Hugh Laurie played the father in the film Stuart Little, American accent included. There's also his role as Internal Affairs Captain James Biggs in Street Kings (in which his character is remarkably similar to House, sans beard or bad leg).
In the original cast recording of Mamma Mia! the travel writer's voice is considerably louder and more "Texan" then the other two Disappeared Dad characters. The fact that the musical originated in England probably has something to do with it. Getting back to the film itself, the Irish Pierce Brosnan played an American character, using essentially his real accent.
He did such a fine job playing Major Cabot Forbes in Glory that he probably should have been cast as the lead given that he resembles the real Colonel Robert Gould Shaw a lot more than Matthew Broderick does.
He did a hideous Midwesternish-Southernish accent for Twister though, with just a hint of still being British.
He did a great southern accent in Kissthe Girls as Det. Nick Ruskin but later when he is revealed to be the bad guy and drops the southern accent for a deeper "standard" American one he slips back into his British accent almost immediately.
In The Crush, his accent slips to British many times throughout the movie.
In Days of Thunder, Cary Elwes puts on a decent Southern accent for his role as Russ Wheeler.
He puts on a less-than-impressive Newscaster American accent for The Chase. Luckily for our ears, it was only a cameo.
Ewan McGregor was technically using a Southern accent in Big Fish, but the result was not authentic-sounding at all. Ditto for his Midwest American accent in The Men Who Stare at Goats. He also played American rocker Curt Wild in Velvet Goldmine. And Oliver (with an American accent and American parents) in Beginners, which was filmed in Los Angeles. He uses both a fake American accent and his natural Scottish accent in The Island.
Virtually everyone except the main cast in the Film of the BookBridge to Terabithia was a New Zealander playing an American. The singing scenes had to be dubbed by Americans because the child extras sounded like New Zealanders.
Jim Sturgess adopts a hilariously bad American accent in 21, and completely forgoes holding his Rs throughout the second half of the movie.
Simon Pegg puts on a fairly convincing American accent in Big Nothing.
Used to varying effect in Bugsy Malone: The movie was filmed in England and used local children for bit parts.
Brian Cox in Super Troopers portrays the Irish-American Captain John O'Hagen, whose accent wobbles between the generic American and Irish depending on his mood, but never sounds Scottish.
While the lead character is played by an American playing a Brit, all the American characters in Sherlock Holmes (2009) are played by Canadians. You can really tell with their accents, especially against the British ones.
South African Sharlto Copley and Irishman Liam Neeson are both Fake Americans in the film version of The A-Team. Copley's wanders through several different Southern accents, but given that it's Murdock it fits quite well, and he deliberately fakes a lot of other throughout the movie. Neeson's more or less works as well, though his brogue tends to slip a bit whenever he says anything with an "oo" sound (such as when he tells Face, "You are really tan.")
Isla Fisher, born in Oman to Scottish parents and raised in Australia, only ever seems to play Americans. This is somewhat justified, however, as if she ever did use her real accent, the result would be so cute, the universe would collapse in on itself.
German movie Locked Up features the main character meeting and falling in love with the "American" Mike while in prison. Mike is just a British guy who has apparently never heard an American speak and wagers no Germans have either. He doesn't even make a flimsy, half-hearted attempt to use anything but his normal speaking voice. Must be a graduate from the Sean Connery School of Accents.
Radha Mitchell (Australian) and Joe Anderson (English) did passable American accents in the Iowa-set The Crazies (2010).
Brit Emily Blunt used a convincing American accent to play Amy Adams' sister in Sunshine Cleaning. She puts on a Southern accent for Looper and plays American again in Sicario. She seems to be averting this and is allowed to use her natural accent for her more famous roles in The Adjustment Bureau, Edge of Tomorrow and Into the Woods. Ironically she is now an American citizen, and her voice has a few American pronunciations from years of working there. Notably in Edge of Tomorrow, where she's using her natural voice, she alternates between English and American pronunciations.
Aussie Emily Browning in A Series of Unfortunate Events. She did initially audition in her role as Violet Baudelaire in a British accent, although it eventually winded up in her using an American accent. Browning continued to use an American accent for her role in Sucker Punch. The same film also featured Aussie actress Abbie Cornish as the American Sweet Pea.
The 1948 film No Orchids for Miss Blandish was made in the UK but is full of fake Americanisms—left-hand drive cars, a vaguely New York setting, and American accents of varying quality. The original novel was written by an Englishman (James Hadley Chase, who wrote a number of novels set in America - although he never lived there) but so thoroughly riddled with Americanisms that English readers mistook it for an American import.
City Island has Emily Mortimer taking part in the exact same twist she did in 30 Rock (that she's "really" an American pretending to be British). She goes the other way around in The Fifty First State as a Liverpool girl Dawn posing as an American assassin Dakota. She plays Americans in Scream3 and Shutter Island too.
Ralph Fiennes (English) playing the American Charles Van Doren in Quiz Show. He uses more of a Mid-Atlantic/preppy New England accent, though, with notable English influences. Mark Van Doren, father of Charles Van Doren, was played by Paul Scofield, also English.
Ryan Reynolds has one role that takes this trope in ironic ways: in The Proposal, he's the guy targeted for marriage by his Canadian boss (played by half-German Virginia native Sandra Bullock) before she loses her visa. Ryan claims he trained himself to lose his Canadian accent to make playing Americans easier.
A peculiar case in My Big Fat Greek Wedding. Nia Vardalos is indeed of Greek ancestry, as is her character, but unlike her character, a Chicago native, she is a Greek-Canadian from the prairies.
The Irish Colin Farrell played not only an American, but an American Country Singer in Crazy Heart.
For the handful lines Anthony Head actually speaks in Repo! The Genetic Opera he puts on a fairly convincing American accent. He slips a bit when he sings, though.
The exploitation film American Kickboxer 1 is false advertising on three counts; it wasn't the first of a series, it was filmed in South Africa with the cast trying and failing to sound American, and as for the kickboxing...
Aquamarine was set in Florida but filmed in Australia (a change from the novel, where the plot involves one of the main characters moving to Florida - in the movie she's going to Australia), and with the exceptions of Emma Roberts, Sara Paxton, Joanna Levesque, Arielle Kebbel and Jake McDorman, the cast affected US accents. Some of them better than others.
Man of Steel has British Henry Cavill was the first non-American actor to play Superman (who, while technically an alien, was raised in America and is considered an American icon). Fans were initially unhappy but most agree he did a very good job with the accent. Lampshaded in the film when he claims "I grew up in Kansas. I'm as American as they come". Cavill once again played a Fake American in the film version of The Man From Uncle 2015. For bonus irony, the real American played a Fake Russian.
Scotsman Alan Cumming had a bit part as an American hotel clerk in Eyes Wide Shut. He tells an awesome story of how director Stanley Kubrick was first annoyed upon discovering on the day they were to shoot his scenes that Cumming was Scottish and not American. Kubrick grumbled: "You were American on the tape!" Cumming coolly responded: "I know. That's because I'm an actor, Stanley." Cumming's chutzpah earned the notoriously intimidating director's respect.
Australian Simon Baker in Margin Call, though it's not always ideal (Given that the film has two British characters and it wouldn't be unrealistic for an Australian to be part of the mix as well in a Wall Street investment bank, it's not clear why it was necessary).
British Joe Anderson pulled off a very convincing American Accent in Across the Universe. Amplified by most of his scenes taking place with fellow Brit Jim Sturgess who plays an Englishman.
Punisher: War Zone has Irish-born Brit Ray Stevenson as the titular New York native, putting on a pretty convincing accent. The film also has Brits Colin Salmon doing a pretty good job as FBI Agent Butanski and Dominic West with a ludicrously over the top Italian-American gangster accent.
Small Apartments has Brits Matt Lucas and Juno Temple and Australian Rebel Wilson as Los Angeles natives. They all do very good accents.
Rebecca Hall, like Christian Bale and Kate Winslet, seems to be building her career on this trope as evidenced by her accents in Vicky Cristina Barcelona, The Town, Lay The Favorite, Iron Man 3 and Christine. Her American accent is so effective most viewers would never guess she is actually British. Iron Man 3 also has the Australian Guy Pearce as Aldrich Killian.
Confederate Army soldier Pencroft is played by the very British Percy Herbert in the film adaptation of Mysterious Island.
The RoboCop remake stars Joel Kinnaman (Swedish of American descent) as Alex Murphy, who is married to Abbie Cornish (Australian). He is turned into RoboCop by the above mentioned Gary Oldman, who works in the same company as Jay Baruchel (Canadian, born in Ottawa and raised in Montreal).
In W. consummate American political insider Karl Rove is played by Briton Toby Jones.
Stoker is rife with fake Yanks, with lead roles played by Australians Mia Wasikowska, Nicole Kidmannote U.S. born but her offscreen accent is pure Aussie and Jacki Weaver as well as Englishmen Matthew Goode and Ralph Brown.
Cara Delevigne plays the American Margot in Paper Towns, but is British in real life. She also does an American accent as June Moore in Suicide Squad (2016).
Suicide Squad also features another example from England, Adewale Akinnuoye-Agbaje (Killer Croc), as well as an Australian (Margot Robbie/Harley Quinn), a Swedish-American (Joel Kinnaman/Rick Flag) and a Native Canadian (Adam Beach) playing a Native American (Slipknot).
Almost the entire main human cast of Terminator Genisys is fake Americans: Aussies Jason Clarke and Jai Courtney respectively as John Connor and Kyle Reese, British Emilia Clarke and Matt Smith as Sarah Connor and Skynet itself respectively, and Nigerian Dayo Okeniyi as Danny Dyson. The only American actor in the main cast is J. K. Simmons.
The cast of The Maze Runner is mostly British, with only Thomas Brodie-Sangster getting to keep his accent as Newt.
The Divergent series is set in Chicago, and yet half of the main cast are played by actors from somewhere in the British Isles:
The Eaton/Johnson family is made up of Fake Americans. Theo James (Four) is English, while Ray Stevenson (Marcus) is Northern Irish. Naomi Watts (Evelyn) was born in England but raised in Australia.
Other English actors include Kate Winslet (Jeanine), Ben Lloyd-Hughes (Will), Suki Waterhouse (Marlene), and Janet McTeer (Edith Prior).
Jai Courtney (Eric) and Keiynan Lonsdale (Uriah) are both Australians.
The Divergent Series: Allegiant adds non-Anglophone actors into the fake American list: the Swedish Bill Skarsgård (Matthew) and the German Nadia Hilker (Nita).
The Lovely Bones has an American family where the mother is played by the British Rachel Weisz and the daughters by Irish Saoirse Ronan (mentioned above) and the New Zealander Rose McIver. Ruth is also played by a New Zealand actress.
Bastille Day features Idris Elba (English), Richard Madden (Scottish) and Kelly Reilly (English) playing three American characters. All the more noticeable because for all intends and purposes they are the only American characters in the movie. Everyone else is French and played by French actors. Richard Madden joked that it was easier for the crew to understand him when he spoke in his Fake American voice rather than his natural Scottish.
Pacific Rim has English man Charlie Hunnam faking an American accent to play the protagonist Raleigh. And his twin brother at the start.
The film Loving depicts a Maligned Mixed Marriage between two people in the American south. They are played by Australian Joel Edgerton and Irish Ruth Negga.
Seventh Son is set in a fantasy world but features a few actors using American accents. Ben Barnes and Kit Harrington (both British) and Alicia Vikander (Swedish).
The Best Exotic Marigold Hotel's sequel features an undercover hotel inspector who's posing as English guest Lavinia Beach but is really American. The character is played by Tamsin Greig, who uses her own English accent while Lavinia is undercover.
The film adaptation of The Girl on the Train changes the setting from Britain to America but ironically two Fake Americans in the cast; Welsh actor Luke Evans as Scott, and Swedish actress Rebecca Ferguson as Anna. The British Emily Blunt gets to keep her accent as Rachel.
Crimson Peak has Mia Wasikowska (Australian) and Charlie Hunnam (English) as the American leads. Jessica Chastain is the opposite - American playing British - as Lucille (though she was approached to play Edythe first).
Bill & Ted: Ironically for being quintessential Californian dudes, neither Bill and Ted are played by American-born actors; Alex Winter was born in England (though moved to the US as a kid, making him a borderline example), and Keanu Reeves is Canadian.
Self Less features Canadian Ryan Reynolds and British Michelle Dockery as American characters.