Heather Locklear and Perry King have already played together in Melrose Place.
One of the classic movies Tracy mentions when bonding with Charlie at lunch, "Apocalypse Now," starred Charlie's father Martin Sheen. He later discusses his father (although actually talking about the plot of the "Lion King") during therapy. In a later episode, Martin Sheen plays Charlie's father.
During the bumper footage before Catlin confronts Charlie the twin towers can be seen despite the episode being aired well after 9/11.