A: Mary has bought a new Porsche without telling her husband.

B: Really? I would love to be a fly on the wall in her house when he finds out about it.

Note: This is an American idiom originally referring to real houseflies on the wall, observing events at the dining table but making no moves. Metaphorically, it often refers to one who is able to observe something closely but invisibly and without interfering in the situation. This expression is often used as an adjective, as in a fly-on-the-wall documentary, where it refers to a filmmaking technique in which events are merely observed and presented realistically with minimum interference, rather than acted out under direction.