stand in

Short definition from Wiktionary

Noun

1. 1
   A substitute; a replacement.
2. 2
   A person of similar size and shape to an actor who "stands in" for that actor during the lengthy process of setting up a shot, but who, unlike a double, does not appear in the film.
3. 3
   A substitute.

Verb

1. 1
   To substitute for; to replace; (theater) to serve as an understudy.
2. 2
   To make one of a party in a bet or other speculation.
3. 3
   To take a side in a dispute.
4. 4
   To cost (an amount).