The term "fly-on-the-wall" refers to a concept where someone observes a situation without actively participating in it. There are several synonyms for this word, including observer, spectator, silent witness, and bystander. An observer is typically someone who watches and takes notice of a situation without getting involved. A spectator is a similar term, referring to someone who is watching a particular event or situation unfold. A silent witness is someone who witnesses a situation without expressing their opinion or involvement. Lastly, a bystander is someone who is present at an event or situation but does nothing to participate or intervene. All of these terms can be used interchangeably with the term fly-on-the-wall.