The phrase "fill out" means to complete or provide information on a form or document, but there are many synonyms to use in its place. Some common substitutes include "complete," "fill in," "answer," "populate," "enter," "record," and "supply." These terms can be used to add variety to writing or speaking, making it more engaging and interesting. For example, instead of saying "please fill out the survey," you could say "please complete the survey questions." Choosing the right synonym can depend on the context and tone of the communication, but using alternatives to "fill out" can help prevent repetition and add clarity to your message.