According to Jeff Bridges, the movie had absolutely no screenplay. Bridges said that the movie was rushed into production to meet its release date, and the director and star just made up scenes as they went along. Bridges said that all they really had was an outline.
The actors and production team would show up for big scenes every day and wouldn’t know what to say. The actors would go into their trailers and work on the scene, calling writers for ideas while the crew was waiting for them to come and shoot.
According to Bridges, full scenes were improvised by Robert Downey Jr. and director Jon Favreau, and the entire production was saved by their improvisation skills. Though Downey Jr. and Favreau took the lack of a finished script well, Bridges was freaked out by it.
If you know much about acting, then it shouldn’t surprise you why such heavy improvisation worked so well in the film. Oftentimes, improvisation can make the characters feel much more real to the audience, because the actors are in character, making up lines on the spot that their characters would say.
Improvisation is to credit for much of the good in certain movies, such as the scene in The Empire Strikes Back when Princess Leia tells Han Solo that she loves him, and Harrison Ford improvised his famous response: "I know." Know any others? Let us know in the comments