Obviously, when you see Tony Stark jump off the roof of an exploding building, that's not really Robert Downey Jr. -- stunt doubles and CGI are there to make sure the franchise's $50 million star doesn't accidentally get whipped in half by Mickey Rourke. But what you don't realize is how little of the star is actually in the movie. When they show anything that isn't the actor's face, there's a good chance the person you're looking at is someone like the stand-in we spoke to.
For instance, the hands shifting gears in the Fast and the Furious movies? All of the males were those of our actor, Thrust Neckpunch (again, not his real name). You're not going to notice that the hand that's only on the screen for half a second doesn't really belong to Vin Diesel, even if it isn't soaked in the blood of the thousands of people he's killed over his movie career, and Vin's time is too valuable to sit there and film a bunch of gear changes. So, they just grab a stand-in whose hands sort of maybe look like Diesel's in the right light.
That scene in Iron Man where Tony is being marched around by terrorists and has a bag over his head, clutching a car battery to his chest? Someone in charge of casting realized "Hey, if we put a bag over his head, we don't have to pay Robert Downey Jr. to do this shot. Someone get Thrust Neckpunch over here!" The same thing happens later when a reporter hands him photos of Stark weapons being used to murder innocent people. When the camera pans down to show him flipping through the photos, those hands don't belong to Robert Downey Jr.
It's also done for convenience, too. The camera angle required for the photo shot would've required Downey Jr. to stand bent over backward for hours at a time so the camera could fit where his head should be, and that's not something you subject an A-list actor to, even if you are paying him enough to actually become Iron Man in real life, should he so desire it.