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.
You can tell it's Vin Diesel's real hand by his two extra thumbs.
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.
Thirty seconds of filming his hands costs more than most small businesses ever make.