Jensen Ackles, who plays C.J., is in an episode of the TV show "Supernatural" with the exact same episode name (The Song Remains the Same).
Kerr Smith and Jensen Ackles played rivals on My Bloody Valentine (2008)
Melissa Ponzio has previously played Dr Tom Frost's girlfriend (Jen's Therpist's girlfriend) in season 4 episode 16. She was a poet that Jack and Jen went to see because they though Dr Frost would be there too. Here she is playing one of the women interested in Emily's flat.