understudy
verb (used with object)
to learn (a role) in order to replace the regular actor or actress when necessary.
Used to express actions, events, or states that have not yet occurred but are expected to happen.
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
You're to have a small part in a new comedy we're putting on next September," he announced, "and at the same time you will understudy the star—Nella Cardrow in 'Mrs. Mixer.'
From Joan Thursday by Louis Joseph Vance