The phrase "fill in for" refers to temporarily replacing someone or something. Some synonyms for "fill in for" include substitute, stand in, take over, cover for, relieve, and hold the fort. Other possible replacements for "fill in for" may depend on the specific context in which the phrase is used, such as taking up the slack, covering a shift, or acting as a deputy. It is essential to select the appropriate synonym for "fill in for" based on the situation to ensure clear communication. By using synonyms for "fill in for," one can add variety to their communication and prevent repetitive language.