1. fly on the wall - Noun
2. fly on the wall - Phrase
fly on the wall (plural flies on the wall)
(idiomatic) A quiet, non-participating, or unseen observer; an eavesdropper or witness.
I would trade all the advantages of humanity to be a fly on the wall when Franklin and Jefferson discussed liberty, Lenin and Trotsky revolution, Newton and Halley the shape of the universe, or when Darwin entertained Huxley and Lyell at Down. Stephen Jay Gould
I wouldn't mind being a fly on the wall in a few Victorian parlours. Sarah Waters
I never shot on sets, but if I was traveling somewhere or on location, I would always have my camera, and I'd always be - it's that kind of fly on the wall approach to photography, though. I don't engage the subject. I like to sneak around, skulk about in the dark. Jessica Lange
No, I was an unknown when I walked in that room. He didn't know who I was from a fly on the wall. Shia LaBeouf