Understudy

[Sarah]

To study, as another actor's part, in order to be hissubstitute in an emergency; to study another actor's part.

[Sarah]

(n) a substitute actor

(vb) to study for the role of another actor, in case they cannot perform their part

[Sarah]

When you have the intention of hooking up with someone but by the end of the night you have to settle for your second choice.

[Dorothey Allen]

(in the theatre) a person who learns anothers role in order to be able to act at short notice in their absence.