To be unnoticed while listening and observing others.
I wish I could be a fly on the wall and hear what he says to his boss when he tries to explain why the project failed.
Tom never came home to his girlfriend last night. I wish I could have been a fly on the wall when he told her the reason.
Example #3: The rival company is going bankrupt. If I could be a fly on the wall at their meeting I could learn what is going on there.
Example #4: They made the documentary with a fly on the wall style.
To be a fly on the wall: This idiom uses the image of the small but silent fly resting on the side of a wall, and observing while human activity continues in the room. The definition is to listen and watch others closely but they don’t notice your presence. To observe a situation as if you are invisible.
In example #1, the speaker wants to hear how the co-worker is going to explain his failure and how the boss will respond to him.
In example #2, the speaker is interested to hear the boyfriend’s excuse and his girlfriend’s response and would have liked to hear how they interacted with each other.
In example #3, the speaker is from a rival company and wants to learn what kinds of internal difficulties the company is facing without them knowing that he is listening.
In example #4, a fly on the wall documentary is an observational style of documentary filmmaking in which the camera records participants doing things in their normal way and responding to situations in their normal way. The style avoids using interviews. The effect is such that the camera, and therefore the viewers, too, are like flies on the wall observing the situation.