Jon Favreau Used Robert Downey Jr. and Gwyneth Paltrow's Real Banter in “Iron Man”: 'I Would Take Notes' (Exclusive)

"They complement each other so well," actor-director Jon Favreau says of the costars

Even before Robert Downey Jr. became a bankable star and Oscar winner, Jon Favreau could see the actor’s impact on any film was electric.

“Robert is always interesting,” he tells PEOPLE. “That is a superpower.”

Which is why Favreau cast Downey as the lead in 2008’s Iron Man, a movie that would cement the star’s leading-man status and launch the multi-billion-dollar Marvel Cinematic Universe.

Part of the film’s success can be credited to Downey’s chemistry with costar Gwyneth Paltrow. He plays Tony Stark, a genius inventor who becomes the superhero known as Iron Man. Paltrow stars as Pepper Potts, Stark’s whip-smart assistant.

The breezy back-and-forth they have in the movie is reflective of the actors’ real conversations — in part because Favreau incorporated it into the film.

“I was a very good student where I would take notes as they would talk, and whether it was improvising or even just talking about the scene, I would write stuff down that they would say,” he recalls.

Related: Robert Downey Jr. Reflects on His Hollywood Journey After Oscar Win: 'There Are Ways to Heal' (Exclusive)

“Gwyneth corrected him once because we were reading the script and it said, ‘This looks like Jackson Pollock's spring period,’ ” says Favreau. “And then she corrected us in rehearsal.”

Continues Favreau about Paltrow, “She says, ‘No, it's actually The Springs period. The Springs is the part of the Hamptons where Jackson Pollock lived and worked, not spring, not the season.’ ”

“So I wrote down every word, and she's correcting him in the movie when he says, ‘the spring period.’ So it just adds to their sort of nippy banter. I find it thoroughly compelling and entertaining to watch the two of them on screen together because they had this banter that has an edge to it. "

“They complement each other so well,” he adds. “They're so different, but yet [have] so much mutual respect and admiration. I love that.”

Related: Robert Downey Jr.'s Wife Susan Shares the '2-Week Rule' That Keeps Their 18-Year Marriage Strong (Exclusive)

Favreau, who also worked with Downey on Iron Man 2 and Chef, says the Sherlock Holmes star is dedicated and always comes prepared.

“He has a tremendous amount of respect for the profession, and he wants to make sure that he's doing his part,” says Favreau. “It’s game day.”

That’s part of the reason it’s so gratifying for Favreau to see his friend sweep awards season for playing scheming government bureaucrat Lewis Strauss in Christopher Nolan’s Oppenheimer.

“It’s nice that the sun is shining on him, because it takes turns,” says Favreau. “You really get the sense that he’s putting things in perspective, and he seems to be understanding what this moment means.”

For more People news, make sure to sign up for our newsletter!

Read the original article on People.