Some element of a work, be it certain dialogue or the classic clothing of a character, wasn't chosen by the creators or the responsible department of the crew, but by the actor. Maybe they walked into set one day wearing leather pants and the director thought the look "just worked" for their character when the possibility had never been discussed before. Maybe they suggested that adding some lines to conversation would be beneficial to their Character Development. Something an actor does with the character which the producer likes either before or after they get the role would also count. These may be examples of Cast the Expert. This excludes improvisations that were included in the final cut, as some precognition and crew approval is polite when you're about to overthrow their production. This is something from Real Life often portrayed behind the scenes of the Show Within a Show In-Universe. May overlap with Actor-Shared Background, and sometimes Throw It In!. Compare The Cast Show Off, when an actor uses some of their Real Life skills in their performance; Harpo Does Something Funny, when the writers understand that this is inevitable; The Danza, when the actor names their character after themself. When an element of a work is inspired by fans or WMG, it's Ascended Fanon.
- After getting the role of Mr Magorium in Mr. Magorium's Wonder Emporium Dustin Hoffman gave the character a speech impediment, because why not, he's already good-crazy.
- Pirates of the Caribbean - Jack Sparrow's way of talking and behaving was basically Johnny Depp adding to a character that was far more low-brow in concept.
- During the open casting call for The Last Airbender, creator M. Night Shyamalan received a video of Noah Ringer doing performance martial arts. He never originally thought about casting Ringer (saying that he thought "wow, that's cool", but didn't originally intend to have so much of it in, or not quite so awesome), but then called him in for some acting auditions and screen tests to see if he could double anyway, and was apparently so impressed by the boy, who does look a lot like Aang as well, that he was hired.
- In Blade Runner the 'Tears In The Rain' monologue was written by the actor, Rutger Hauer, when he didn't like the original lines. Notably, the modifications included the "like tears in the rain" addition at the end.
- In Full Metal Jacket, R. Lee Ermey turned Gunnery Sergeant Hartman from a stern but understanding officer (read:unmemorable character) into the trash-talking debasing Drill Sergeant Nasty that has now become iconic.
- Apparently Robert Downey, Jr. kept hiding/eating snacks on the set during certain scenes of The Avengers, so eventually it was incorporated into the film; "Blueberry?"
- In Star Wars, the famous violet Lightsaber used by Mace Windu was the suggestion of his actor, Samuel L. Jackson.
- Pitch Perfect: Beca's singing "When I'm Gone" as her audition piece was because Anna Kendrick already knew that song, having seen it on a Viral video and spent an afternoon learning it.
- The Addams Family: It was Christina Ricci's idea to have Wednesday Addams fold her arms on her chest like a corpse being laid out for a funeral when Wednesday was being tucked into bed. Ricci was about 10 at the time.
- 24: Sarah Clarke auditioned for and was cast as Nina just one day before filming began and the wardrobe department didn't have time to give her a fitting, so she ended up wearing what she wore to the audition as her costume. Due to the premise of the show occuring over the course of one day in real time, she wore her own outfit for the rest of the season. Differs from Throw It In! in that it was at Clarke's suggestion.
- In Star Trek: The Original Series, Leonard Nimoy came up with the Vulcan Nerve Pinch, the Vulcan salute, and the "Live long and Prosper" salutation.
- Most, if not all, actors taking the lead role on Doctor Who have had at least some input into their costume design and The Nth Doctor's idiosyncrasies.
- Several CSI characters were renamed by the actors. William Petersen chose Grissom's last name because he was a space program fan (there was an astronaut Grissom.) Gary Sinise named Mac Taylor, who was to be called Rick Castelluci, after his son, McCanna "Mac", and his Forrest Gump character, Lt. Dan Taylor. It was also Gary's idea for Mac to have lost his wife on 9/11.
- Branson of Downton Abbey was going to be from Yorkshire but actor Allen Leech tried in both that and his native Irish accent. Lord Fellowes decided he liked this and it became the character's entire identity, the change in nationality basically writing its own subplots for the last four series.
- In Bewitched, the reason Samantha wiggled her nose to cast spells is because actress Elizabeth Montgomery had the very rare ability to wiggle her nose.
- The Scrubs Season 1 DVD commentary mentions that Zach Braff (JD's actor) was the one who suggested the theme tune be Superman by Lazlo Bane.
- Eddie Riggs in Brütal Legend was at first designed as a general roadie with a hint of Deadpan Snarker, and he even took after Lemmy for a while. As the character was developed, elements of Jack Black were thought of, but the designers couldn't have guessed that Jack Black himself would approach them to work on the project (he was a fan of Double Fine's previous work, Psychonauts). Eventually, Eddie became Jack.
Hello, Unknown Troper. You'll need to get known to lend a hand here.