As far as the supporting cast goes . . .
Howard Stark, Tony's father, is supposed to have a vital role. Having just watched King of New York, I can't help but think
Christopher Walken for the part. He's a great actor who has drawing power.
Obadiah Stane has been rumored to be vital as Stark's corporate opposition. For the villainous CEO, I'd cast
Clive Owen. He's one hell of an actor who can be downright villainous.
Happy Hogan is Stark's friend and confidant. For the comic relief, I'd cast one of two people. If you want a star, get
Scott Caan. He's great at comedy and drama and really looks the part.
If you want a smaller actor who can also be great for comic relief, hit up
Jeff Anderson. He's been nominated for an Independent Spirit Award thanks to his role of Randal in Clerks.
Pepper Potts-Hogan is another of Stark's friends who works for Stark Industries. It isn't a big role, but it can be important for Stark to have true friends in the midst of his business dealings.
Kadee Strickland is the beautiful star of Anacondas who is not very well known, so she'd be cheap to get.
Finally comes
Jim Rhodes. There are a lot of great actors who could take the role. I lean toward Boomtown's Mykelti Williamson. He's a good actor who is big enough (6'1) to intimidate.
