fly on the wall

I remember I heard it first in T.A.T.U. song:

When you're naked in the shower
When you're sleeping for an hour
When you're big, when you're small

Oh, I wish I was a fly on the wall!

Actually, there is no much hidden meaning behind this line. If you refer to someone as a fly on the wall, you mean that this person is an unnoticed observer of a particular situation, or just someone who secretly watches something, probably witness at a crime scene.

People still like this expression a lot. There are many other songs using it, there is a novel called the same, even The Beatles gave one of their releases this name.

Or, for instance, you might hear about fly-on-the-wall documentary, which is a film-making technique whereby events are recorded realistically with minimum interference rather than acted out under direction.