n. 预备演员, 替角

vt. 通过观察掌握某工作, 通过实习掌握某工作
vt.vi. 练习当替角

Understudy \Un"der*stud`y\, v. t. & i. (Theater)
To study, as another actor's part, in order to be his
substitute in an emergency; to study another actor's part.


Understudy \Un"der*stud`y\, n.
One who studies another's part with a view to assuming it in
an emergency.