Like its hero “Iron Man” takes false steps, stumbles, and even occasionally crashes, yet quickly recovers its footing.

The reason it’s so nimble is that director Jon Favreau (“Elf,” “Zathura”) and his fleet crew of actors grasp the action-fantasy premise and treat it with the looseness and sharpness of improvisational comedy. (Favreau himself has worked out with The Groundlings troupe in Los Angeles from time to time.) It’s difficult to tell how much of what they’re doing is taken directly from the script (credited to four writers, and who knows how many others labored behind the scenes), but even when they’re reciting somber dialog-bubble exposition, they treat it the way an improv actor would: smoothly feeding information into the scene, building a foundation on which everybody can work, and play.