Downloaded and have been listening to the soundtrack to Dr. Horrible's Sing-a-Long Blog. I recently, in conversation, cited Chicago as a great movie musical because they used the movie effect of placing all the songs as Roxy's fantasy version of events.
DHSaLB, on the other hand, and which is a pretty perfect movie musical, doesn't do that- aside from the Bad Horse epistles, all the songs are rooted in the "reality" of the story; BUT, I never noticed till listening to the songs divorced from the story, almost none of the songs actually END. All except "My Eyes" (or, as I thought it was titled, "On the Rise") are interrupted by a plot event, and ALL the songs move the story forward in some way. Which makes sense, since DHSaLB was actually written FOR film, there is no live audience to applaud, there's no need for a button at the end of songs, and so we can just proceed with the plot.
Ties in to something I noticed about In The Heights (a pretty fantastic stage musical)- despite the "radio edits", there is very little from the show that's actually excerptable, because so much of it is plot-based. Even Vanessa's I Want Song gets interrupted by Usnavi and Sonny and I need some packing tape coño!
Things to bear in mind while writing the musical I'm working on now.