Stand-ins are distinguished from doubles, who replace actors on camera from behind, in makeup, or during dangerous stunts.
For instance, a scene in Heidi (1937) features the title character's grandfather (Jean Hersholt) mistake a stranger for Heidi (Shirley Temple); the look-alike was played by Temple's regular stand-in, Mary Lou Islieb.
[1] In another case, the actress who pretends to be Ann Darrow in the stage show during the final act of King Kong (2005) is played by Naomi Watts' stand-in, Julia Walshaw.
Famous cases include Pluma Noisom (stand-in for Claudette Colbert), Harry Cornbleth (Fred Astaire) and Adam Bryant (Robin Williams).
When Bette Davis walked out on her Warner Bros. contract, she negotiated for her regular stand-in, Sally Sage, to continue to work at the studio.