Shawn was pitching a musical to Z. In a musical, the characters, when they sing, aren't aware they are singing. The opening number (and reprise), the Tango (and resprises), and Kurt Fuller's song were all musical style, something is happening, and then people break into song. The times where they were aware people…

If it would have been a dream, people would have complained it was a cliche to do that (and they're doing a dream episode next season anyways). Some of the songs were forced by Yang, so they have legitimate explanations. The song between Yang and Mary is a dream sequence, so I think that covers that one. There had to…

They filmed the season already, so she won't be in any more episodes than she already is. It was all filmed before Back in the Game even starting airing.

Originally this episode was supposed to air before Juliet found out, but the USA Executives wanted it to air separately from the season so, although a nice idea, it was never going to happen that way.

It was explained at the end. Shawn and Gus, inspired by the musical production they came across while solving the crime and making friends with Z, tried to pitch him to write the crime they solved into a musical.

To be honest, most musical episodes of any show bring in some Broadway star and give them a song or two, but the majority of the songs go to the main cast. And why not, it's their show. You want to see the guest star sing? I'm sure there are other places out there you can do that. Could they have given Rapp more?…