Other forms: flies on the wall
If someone wants to be a fly on the wall in a situation, they want to be someone who can watch and listen to what's happening without being noticed. In other words, they're curious and want to spy on people!
You may hear this expression used in sentences like "I’d like to be a fly on the wall when her parents find out what she did" or "Don’t you wish you could be a fly on the wall for that conversation?" People use this phrase when they really want to know what was said or done by other people, but realize they can never ask or would never be told. In what situations would you like to be a fly on the wall?