Excerpt from Michael Caine’s awesome book Michael Caine – Acting in Film: An Actor’s Take on Movie Making 
Sometimes lighting a scene can take an hour and a half to two hours, and no one expects the actor who’s in the scene to stand there that long. My stand-in is usually a big 6’2″ blond guy whose face is the same height from the floor as mine to ensure that when I get back on the set my face won’t be in the dark. Stand-ins will sometimes get you a cup of coffee; my stand-in is a particular friend of mine. But stand-ins can on occasion send you rushing out to look at yourself in the mirror.
I’ve been in movies now for close to twenty-seven years, and when I first started, they’d say, “This is your stand-in,” and there’d be this great-looking young guy standing there. Eventually, one morning you come in and they say, “This is your stand-in,” and there’s this old fellow standing there with a bald head, wearing a wig.
