Paul Bettany:"I used to be in a studio for 45 minutes and do J.A.R.V.I.S. and get a huge bag of cash and go my way like a burglar, and now they want me to work for my money. Which is great and sweaty and hot, which you'll realize once they unveil everything. It's really f***ing cool. It's great to join this train which is on really clear tracks."
Don Cheadle as War Machine receives higher billing than Aaron Taylor Johnson (Quicksilver), Elizabeth Olsen (Scarlet Witch), and Paul Bettany (The Vision) despite having a minor role and not even being part of the Avengers (until the end).
Joss Whedon's feelings on working on this film were... mixed to say the least. He has always been a big comic book fan (having actually written for X-Men) and due to his cult status he was allowed more input into the Marvel verse than the other directors. He wanted to make the movie smaller and more personal... and found himself filming a movie on an epic scale across the globe (it just sort of happened), and that meant he had to spend a long time away from his family in Burbank. He explicitly did claim that he felt that the movie should have had a longer running time (closer to his original cut), which is probably why the rumored extended edition is said to be 40 minutes longer. He's since walked back on his comments, saying that he would be happy to be asked back to the MCU.
Chris Hemsworth was unhappy with Thor's role in the movie, feeling it was boring to play. He recalls telling Kevin Feige "I feel like I'm dying here. I feel like I have handcuffs on," which is what ultimately led to the character adopting a more comedic personality in Thor: Ragnarok.
Deleted Role: Tom Hiddleston was meant to return as Loki for Thor's nightmare sequence. However, test audiences found the nightmare sequence confusing (apparently some thought that Loki was controlling Ultron, and the fact that the scepter he used in The Avengers to induce mind control was a MacGuffin in this film probably didn't help), so it was drastically shortened, removing Loki's appearance in the process.
Deleted Scene: Considering the criticisms regarding pacing, some of the deleted/extended scenes do help transition between moments better:
Scarlet Witch and Quicksilver are handing out stolen supplies to the people of Sokovia. Pietro gave Zrinka an expensive dress from Paris and a toy for her brother Costel. Wanda warned Pietro that stealing is dangerous despite his powers, noting that when he uses his speed he cannot be shot. Costel approached and told them that "Iron Man" was waiting for them at the church, in which the twins were confused.
Bruce Banner and Tony Stark examine the Ultron Program.
Von Strucker walks into his jail cell and is killed by an off screen Ultron.
Maria Hill and Captain America discuss Ultron killing Wolfgang von Strucker.
James Rhodes contacted Black Widow and Stark about Ultron hacking into nuclear weapons programs around the world. He is being deployed to the Middle East to help keep the peace. Stark sends him a patch to keep Ultron from hacking into the War Machine Armor. Thor attempts to contact Heimdall but gets no response.
A bustling African village is torn apart by Hulk while he is on his Wanda Maximoff-fueled rampage. A little girl is trying to recover her ball and stumbles across him.
An extended scene of Banner and Romanoff's conversation at Clint Barton's Homestead.
An extended scene of the birth of the Vision, including the eye-opening scene seen in trailers. In this alternate version, Vision has an extended fight scene with Thor and the other Avengers.
Thor and Erik Selvig go to the Water of Sights; they are greeted by a Norn who possesses Thor and answers Selvig's questions through him. The Norn tells Selvig that Ultron has the Mind Stone, one of the six Infinity Stones.
Tom Hiddleston shot a cameo as Loki in Thor's dream sequence. Whedon revealed, "We even had a little reference to the fact that hes taken the throne, which was Tom doing his Anthony Hopkins impression when Thor says, 'Oh, what would father say?' Then Tom does his Anthony Hopkins impression, and Thors like, 'That is uncanny!' Its sort of like his subconscious is telling him that Loki was imitating his father. But he would never make that connection."
Captain America and the Avengers land in Sokovia and see some graffiti that call Captain America a fascist. This motivates him to not wear his helmet in the Battle of Sokovia.
Captain America gathers Scarlet Witch, Black Widow and Hawkeye to battle the Ultron Sentries. Black Widow makes a remark about Scarlet Witch wearing her jacket. The latter suggests that they try to get civilians out of the city before it rises too high. Quicksilver runs up, ready to fight, and the characters take off into the battle.
Quicksilver runs through a neighbourhood of Sokovian balconies, telling civilians to get to safety. He stops to flirt with a girl, asking "Maybe I see you after?"
A scene was shot of Quicksilver surviving his wounds as well as footage of him in a new costume at the New Avengers Facility, in case Disney overruled Joss Whedon's decision to kill him off.
Marvel executives did not approve of Hawkeye's farm or the visions that Scarlet Witch gave the Avengers; in fact they threatened to cut them unless Joss put in the subplot with Thor in the cave. And then they ordered that cut when audiences reacted negatively to it — but the editor told Joss that without it, things wouldn't make sense. So what we got was a mangled version.
The Seoul city government provided handsome tax breaks to the production team. In exchange, Claudia Kim was cast in the minor, but important, role of Helen Cho.
Exiled from Continuity: Magneto is Scarlet Witch and Quicksilver's father in the comics, but can't be mentioned here because of rights issues. Conveniently, the Axis comic revealed that they aren't actually related to Magneto shortly before the release of the film. (Meanwhile, in the X-Men films, Magneto is very much Quicksilver's pa; it'll be interesting to see what retcon hits to tie in with whatever movie puts that at the forefront.)
International Coproduction: The production had segments shot in South Africa, South Korea, and Italy, with involvement and cooperation from the three respective nations. Accordingly, Korean actress Claudia Kim was cast as the supporting character Helen Cho, similar to the Stunt Casting of Fan Bingbing in the Chinese cut of Iron Man 3, although Kim's character is actually integral to the plot rather than completely superfluous like Fan's.
Irony as She Is Cast: Black Widow talks about being sterilized as part of her training so nothing would interfere with her devotion to the mission. Scarlett Johansson was pregnant while filming the movie.
Looping Lines: While James Spader was on-set for the majority of his scenes in a mo-cap suit, B-Roll footage shows him recording some of his lines in a sound booth. This makes the scenes where Ultron suddenly speaks with a different voice understandable.
Lying Creator: Prior to the release of the film, it was said that Thanos would not appear in any other Marvel movie released before 2016. He's the subject of The Stinger.
Missing Trailer Scene: One of the trailers showed a brief glimpse of a dark-skinned bald woman disrobing in a cave. The internet went wild with speculation, with theories ranging from Princess Shuri to Mantis and even Hela. This proved to be completely fruitless, since the scene with the bald woman was cut from the final film.
According to Joss Whedon's DVD commentary, Robert Downey Jr. improvised his "Hello Deere" line when working on the tractor. Whedon liked the line so much that he admits in the same commentary that he kicked himself for not having thought of it.
Tom Hiddleston was set to reprise his role as Loki for Thor's dream sequence, and even filmed the scene in a one day shoot. However, test audiences were confused by his appearance, and mistakenly thought that the dream was meant to imply that Loki was the true Greater-Scope Villain and the one behind Ultron's creation.
According to the actor himself, there were briefly plans to have Tim Roth reprise his role as the Abomination from The Incredible Hulk, but the idea was scrapped.
Carol Danvers appeared in earlier drafts of the movie as Captain Marvel. Marvel feared introducing her without explaining who she was would be too confusing for casual viewers, so they decided to wait and introduce her in her own movie instead. This was changed so late in production that Whedon had actually shot the live-action plates for her entrance. The shots were later used for when Scarlet Witch joins the New Avengers.
Whedon wanted Spider-Man to appear as part of the New Avengers at the end as well, but the deal between Marvel and Sony had not yet been finalized at that point.
Whedon considered that the Avengers would find out Coulson is alive, but he decided against it because he felt it would overshadow the rest of the film.
The subplot with Thor and Erik would've been longer. That cave they went into was a mirror of the Well of Urðr in Asgard. In order to talk to the Norn, Thor had to enter the Pool as a sacrifice and Erik would ask the Norn about the Mind Stone inside Loki's specter and Thor would manage enough willpower to exit the pool before the Norn could consume him.
In addition to Captain Marvel, there were plans to use Age of Ultron to introduce a whole slew of new superheroes. The idea was scrapped fairly quickly for the same reason they ended up cutting Captain Marvel's cameo.
It was rumored early on that an initial cut of the film was nearly as long as The Dark Knight Rises, clocking in at 160 minutes (along with a later report that clarified that the initial cut of movie was over three hours in length). Eventually, it was confirmed that the theatrical release would be 142 minutes. A subplot involving Thor was reduced in length as a side effect of this, and the Loki appearance of the mysterious woman seen in the trailer is not present in the final film. While fans wanted an extended cut, the home video release was of the theatrical version, and very few deleted scenes were included in the extras.
Joss Whedon: It's a great gag, but I couldn't justify it. We were building a lot of the final battle around it, and it was killing us. Even when we were shooting. We had to stutter-step everything else, and eventually in post I convinced them we needed to jettison this concept. I knew I could write a conclusion for Bruce and Natasha that I thought would be much better storytelling, and would be a real moment.
The Hulk was originally going to transform into his Gray Hulk form after being brainwashed by Scarlet Witch. According to the VFX artists, "Everybody wanted the green guy," so they settled for changing his eyes instead. A Gray Hulk figure from Funko was even produced before the character was scrapped.
Whedon actually conceived an alternate ending where Quicksilver survived his heroic sacrifice and was shown as part of the New Avengers line up in case Marvel vetoed his original ending. Concept art was even done up of a new costume for Pietro.
Ultron's fascination with Pinocchio and "I've Got No Strings" was originally going to be Emily Dickinson and "Because I Could Not Stop for Death".
Former Ant-Man director Edgar Wright wrote an after-credits scene for this movie, which didn't make the cut.
Ultron was originally going to steal the vibranium he needed from Wakanda, the home country of the Black Panther. Elaborate concept art of Ultron forging a new body in a Wakandan mine was produced, but failed to make it into the finished movie.
As with any movie, there were numerous alternate designs for the characters done up by concept artists. These include a more comic-accurate take on Ultron with his trademark jack-o'-lantern face, as well as an alternate Quicksilver outfit that included a green shirt with a white lightning bolt as a nod to his original costume.
There was a concept art of the Avengers fighting an Ultron which all got merged into one giant being.
South Korean actor Don Lee (a.k.a. Ma Dong-seok) (Train to Busan), who will be playing Gilgamesh in The Eternals, was asked to appear in a cameo role since he's a well-known actor in the country, worked as a personal trainer for martial artists in the US (he has American citizenship), and the movie was shot in Seoul. He declined due to the scheduling conflict. Good thing is that Marvel haven't forgotten about him and the popularity of Train to Busan among American film goers made him internationally recognizable.
Word of God: Kevin Feige dispelled any rumors that Quicksilver survived in the continuity, stating that the character will not be returning to the MCU for anytime soon.