On the two-hour episode, Bostwick will play Armitage, the owner of a Santa Barbara theatre, and Rapp will play the playwright of an ill-fated musical. Also guest starring on the episode is Brooke Lyons ("Beautiful Boy," "2 Broke Girls").
As previously reported, series creator Steve Franks confirmed at the 2012 Comic-Con that the episode, which will feature all original music, will air outside of the series' regular schedule towards the end of the season.
Here's how the musical episode is described: "Shawn and Gus track down escaped playwright mad man named Z (Rapp), who six years earlier was deemed criminally insane after locking a critic in a back room of the theater that he burned to the ground. Shawn and Gus are forced to consult with the one person Z confided in while in the institution, a dangerous villain from Shawn's past."
"Psych," according to USA Network, "is a quick-witted comedy/drama starring James Roday as young police consultant Shawn Spencer who solves crimes with powers of observation so acute that Santa Barbara PD detectives think he's psychic. Psych also stars Dule Hill as Shawn's best friend and reluctant sidekick, Gus, and Corbin Bernsen as his disapproving father, Henry, who ironically was the one who honed his son's 'observation' skills as a child. Also featured are Timothy Omundson, Maggie Lawson and Kirsten Nelson."
Rapp, who originated the role of filmmaker Mark Cohen in Broadway's Rent, has also been seen in You're a Good Man, Charlie Brown; Six Degrees of Separation; and Precious Sons.
Watch the trailer: