The phrase "stand in" can have a number of different meanings, and depending on the context, there may be a variety of synonyms that can be used to convey the same idea. For example, if someone is serving as a temporary replacement for someone else, they might be described as a substitute or a proxy. If someone is standing in a particular place or location, other synonyms might include occupy, reside, or inhabit. In some cases, "stand in" might also reference a figurative sense of taking someone's place or representing them in some way, which could be described using synonyms like represent, embody, or symbolize.