It’s always a tricky bit when — for reasons necessary to the forward motion of the plot — you have to have one character explain to another character a bunch of stuff that the reader already knows.

This is sort of the opposite of the much commoner “how do I work in the necessary exposition” problem, and tends to crop up in long, complex novels, or in novels where the interlocking plot developments are engineered with clockwork precision, or in parts of an ongoing series.  Unlike the necessary-exposition problem, where the characters already have the required information but the reader does not, with this problem it is the readers who are already in possession of the information, and one or more of the characters who must somehow acquire it.

The trick, to the extent that there is one, is to remember that the reader doesn’t need to be given the information all over again; the reader only needs to understand that the character has been made aware of it.  You can be blunt and direct:  “On Wednesday at lunch, the Director told the Chief of Police everything that had happened during the preceding spring.”  Or you can be more subtle, and have a scene with the Director and the Chief of Police talking over steak and baked potatoes, during which you slip in as much as possible of the known-to-your-reader stuff in indirect discourse, while using the up-front chatter for character and atmospherics and other hopefully-interesting new material.

What you want to avoid is having your reader exclaim, upon  finding out that one of your characters is in possession of a particular piece of knowledge, “But how did he know that?”