Rebecca Hall has said it was inspiring to work with Robert Downey Jr on Iron Man 3.
The British actress plays scientist Maya Hansen in the third instalment of Marvel's superhero series, with Robert reprising his role as Tony Stark.
She told MTV News: "He's a really brilliant improvisationalist.
"He's incredibly quick-witted. Everything he brings to Tony Stark is him. Of course there's all of the tradition from the comic books, the movie, and also the writing is brilliant, but there's also a heavy dose of that character is Robert Downey Jr. "
The Parade's End actress added: "He's inspiring to work with and watch. He's a force."
Guy Pearce and Sir Ben Kingsley are also set to star in the film, which opens in April, alongside Gwyneth Paltrow, who returns as Pepper Potts.