Gregg, who directed Rockwell in this week's Choke, told DS that Iron Man helmer Jon Favreau had Rockwell on his shortlist to play billionaire industrialist Tony Stark.
He said: "Sam was on his early list for Iron Man, he's a big fan of Sam Rockwell's."
Gregg added that he is hoping to reprise his role as S.H.I.E.L.D.'s Agent Coulson in the sequel, though no talks have taken place.
He said: "Not officially, which is why I'm very nervous about it. I have bumped into Jon Favreau, he's given me a couple of winks and nods, like Agent Coulson might make a repeat appearance."
> Click here for our full interview with Clark Gregg