The term 'understudy' is mostly used in theater and performing arts contexts. She's an understudy for the Broadway show.
Pro Tip 2/3
Unexpected Opportunity
'Understudy' often implies being ready for sudden chances to perform, rarely with short notice. He never expected to go on stage, but as an understudy, he was prepared.
Pro Tip 3/3
No Spotlight
An understudy might not receive much public recognition unless they perform. As an understudy, she rehearsed but wasn't featured on the posters.