understudy

Etymology

under- + study

verb

1. To study or know a role to such an extent as to be able to replace the normal performer when required.
2. To act as an understudy (to someone).
3. To act in a similar manner to some known person.
   That lean gentleman had apparently understudied Pharaoh of old and hardened his heart. 1935, Owen Conquest, Morny's Master Stroke

noun

1. A performer who understudies; a standby.
   For Wynne-Edwards, the role of the outcasts in the welfare of the group is to wait in the wings as understudies, ready to step into the shoes of any territory holder who dies on the main stage of group reproduction. 1976, Richard Dawkins, The Selfish Gene, Kindle edition, OUP Oxford, published 2016, pages 153–154