Some follow-up articles have indicated that the role is that of Dr. Maya Hansen, a character responsible for the "Extremis" nanotechnology that is said to be a key part of the sequel's focus.
Originally Posted by Deadline
EXCLUSIVE: I’m told that Jessica Chastain is back in the mix to play a starring role opposite Robert Downey Jr in Iron Man 3.
The role is a sexy scientist every bit as smart as Tony Stark.
Chastain, who last year starred in The Tree of Life, The Help, Take Shelter and The Debt, was originally offered the role but it looked like she was going to be preoccupied making a movie with Joel Edgerton and Samantha Morton.
I think that movie was The Disappearance of Eleanor Rigby.
She also had scheduling issues involving the Kathryn Bigelow-directed Sony Pictures film about the hunt for Osama Bin Laden.
Marvel then began lining up meetings with actresses like Diane Kruger, Gemma Arterton and Isla Fisher.
But all that has changed as Chastain’s movie has been pushed, the Bin Laden scheduling was worked out, and it looks like she’s going to get the role after all.
She will join Downey, Gwyneth Paltrow, Don Cheadle, and potentially Guy Pearce and Ben Kingsley in the third installment of the Iron Man trilogy, this one directed by Shane Black.
Looks like Shane Black is putting together a nice little cast for Iron Man 3!