1. In film and television, someone who stands in place of an actor while lights, cameras, and sound equipment are set up and adjusted. I got a job as a stand-in for a local television shoot. It's pretty dull work, but the pay is good!

2. By extension, anyone who acts as a substitute for the duty or role of someone else, especially on a temporary basis. The vice-president assumed she would be nothing more than a stand-in while the president recuperated from his illness, but after he died, she was forced to lead the country for real. Don't get so high and mighty with us, Jeff. You're just a stand-in while the boss is on vacation.