If you say that you would like to be a fly on the wall on an occasion, you mean that you would like to hear what will be said or see what will happen while not being noticed.
I'd love to be a fly on the wall when those two get home!我想暗中听听那俩人回到家说些什么!
See also: fly-on-the-wall
fly-on-the-wall
adjective[ before noun ]UKuk/ˌflaɪ.ɒn.ðəˈwɔːl/us/ˌflaɪ.ɑːn.ðəˈwɑːl/
(电视节目)纪实的
A fly-on-the-wall television programme is one in which the people involved behave normally, as if they are not being filmed.