I don't know if this has been done before, but I thought it would be fun to try here. Basically we all know the Six Degrees of Kevin Bacon theory (No? Read first link)

I'll start off with Alan Rickman and Emma Thompson. Both actors starred as professors in the Harry Potter films. They also starred together as husband and wife in the film Love Actually.

From that connection, the next person could say that Love Actually also starred Bill Nighy, who portrayed Rufus Scrimgeour in Harry Potter and the Deathly Hallows.

And so forth, and so forth. Basically keep adding some sort of reference from the previous answer and just keep going from there. Just go with it and have fun :]