"Fill for" is a phrasal verb used to describe the process of temporarily doing someone's job or duties while they are away. Some synonyms for "fill for" include substitute, replace, cover, stand in, and deputize. These synonyms emphasize the idea of taking over temporarily until the person returns. Other related phrases include "step into someone's shoes" and "hold the fort." It is important to note that while these words have similar meanings, they may have slightly different connotations, and the appropriate word to use will depend on the context and intended meaning.