- Robert Downey Jr. as Iron Man
- Terrence Howard as James "Rhodey" Rhodes
- Jeff Bridges as Obadiah Stane
- Gwyneth Paltrow as Pepper Potts
- Shaun Toub as Yinsen
- Faran Tahir as Raza
- Paul Bettany voices J.A.R.V.I.S.
- Leslie Bibb as Christine Everhart
- Clark Gregg as Agent Phil Coulson
- Samuel L. Jackson as Nick Fury
- Jon Favreau as Happy Hogan
- Bill Smitrovich as General Gabriel
- Peter Billingsley as William Ginter Riva
- Gerard Sanders as Howard Stark
- Nazanin Boniadi as Amira Ahmed
- Although the story, casting, and action sequences were all planned out by the time that shooting for Iron Man began; very little of the actual script had been written which forced the cast to ad-lib and improvise for much of the film. In fact, Robert Downey Jr. improvised almost all of his lines on the spot which he enjoyed as it allowed him immense creativity in his portrayal of Tony Stark. Gwyneth Paltrow on the other hand found it incredibly difficult as Downey frequently changed entire sections of his speech from take to take which left her groping for a suitable response to his lines.