In which situation would a writer most likely use formal

Depends on the situation and the audience

Lets say a writer is going to a interview or trying to get a job obviously they wouldnt use sland or non formal words cause that wouldnt help so they would youse formal language/words to get them looking presentable or understanding

Or in genral if he or she is talking to people like at a grow. Up party or pub or anything with adults youd assume they would use formal