What is the origin of the idiom "fly on the wall" and when to use it?

The idiom "fly on the wall" originated from the idea of being an inconspicuous observer in a room, able to witness events or conversations without being noticed. This metaphorical expression reflects the desire to be unobtrusive and gain insight into situations without influencing them. The concept likely draws from the notion of a small, unnoticed insect observing events from a strategic vantage point. Over time, the idiom has become a popular way to describe the act of quietly observing or eavesdropping on conversations, often to gain a candid understanding of a situation.