stand in
substitute for someone/something
He was with the national tour of "The Lion King", understudying the roles of Scar and Pumbaa.
She essentially understudies eleven characters in the musical.
Lisa Fennell, in addition to her role in the chorus, was to understudy Désirée.
SMART Vocabulary: related words and phrases
Cinema & theatre: acting, rehearsing & performing
act out
am-dram
amateur theater
appearance
blocking
camp it up
chew
debut
enact
improvise
offstage
portrayal
re-enact
recreation
rep
restage
stage business
stage fright
understudy
unperformable