stand in

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stand in for (someone or something)

To be or act as a substitute for someone or something; to do something in someone's or something's stead. It was decided that I would stand in for my sister at the head of the company while she took her sabbatical. Because of my diet I have this low-calorie spray standing in for olive oil while I cook, but it's just not the same. We stand in for the actors so the crew can light the set properly.
See also: stand

stand in (for someone)

to substitute for someone; to serve in someone's place. The famous opera singer was ill, and an inexperienced singer had to stand in for her. The new singer was grateful for the opportunity to stand in.
See also: stand