Scriptwriter Stephen McFeely has revealed that actors Chris Evans and Scarlett Johansson added to the Captain America: The Winter Soldier script.
McFeely told Metro that he didn't mind the film's stars adding dialogue as they knew their character's voices better than he does.
He explained: "The script was in pretty good shape when we started, but Chris had a lot of input on the scene where he's meeting Sam Wilson for the first time, and what it would take to make Captain America - who probably gets stopped in the street all the time - turn around for that extra conversation.
"And then he and Scarlett were involved in that scene in the car when they're driving to Camp Lehigh and they're talking about kissing and all that."
McFeely added: "I think actors very often know their voices better than I do, so I have no problem with them adding a couple of lines to a scene or telling a different joke.
"It's only when they want to take a scene and make it about something else that it gets problematic. As long as we get what we're trying to accomplish then I don't care."
The third Captain America film will enter production in early 2015.