Robert Downey, Jr. is so determined to make Iron Man 2 a hit, he's helping rewrite the screenplay to ensure it surpasses the original superhero movie.

The actor starred as billionaire entrepreneur Tony Stark in the first comic book blockbuster, which has grossed more than $585 million worldwide since its 2008 release.

And he's playing an active part in the follow-up by reworking the script, according to director Jon Favreau.

Favreau tells U.K. magazine Empire, "Robert is a real partner in the process. He's been very involved in the screenplay. When he went away to do Sherlock Holmes he was still part of our creative process. Even in the first film, where he was originally a hired gun playing the role, he really stepped up to rewrite scenes - he's a great writer, too. So we really are sharing the responsibilities."

And Downey, Jr. insists it's been rewarding to see his hard work pay off, adding, "We are grinding our minds and hearts in the ground on this movie. Could we have just skated through a sequel and would it probably have made the same box-office numbers? Yes, sure.

"I have fortified my belief that if I have a creative instinct about something, usually it's not because I've had too much coffee or because I'm bored - it's because I sense there's something there. And it's always mind-blowing when you follow a hunch and realize it's exactly what the movie wanted."