understudy

Short definition from Wiktionary

Verb

1. 1
   To study or know a role to such an extent as to be able to replace the normal performer when required.
2. 2
   To act as an understudy (to someone).
3. 3
   To act in a similar manner to some known person.

Noun

1. 1
   A performer who understudies; a standby for a study (9).