Student or Learner
Mel is pissed off as a ghost is playing a trick on her making her meet his ex-wife for a useless thing, so she yells like "let me in on the joke". What is "let me in on the joke"? Please let me join in this trick?
Mel-You think this is funny? Making me tell that poor woman
that her husband is dead?
Let me in on the joke, pal. This doesn't mean that i believe any of this.
I think it could be either of the above answers- the secret or unknown information required to understand, or an explanation of where the humour is.