So, I've been thinking a lot about Tim Burton's movie adaptation of Stephen Sondheim's stage musical, Sweeney Todd. (Or, rather, I was recently reminded of a question that I had when it first came out, and that was never quite answered to my satisfaction.)
So... personally, I was always disappointed that they chose not to include any of the lyrics from the "Ballad of Sweeney Todd" (the opening number or any of its reprises). I kinda got why they wouldn't want it to be a chorus number, since they cut out or changed most of the libretto that required a chorus (I believe that there's an interview somewhere out there in which Tim Burton talks about how that was a conscious decision). And there were some lyrics in the "Ballad" that wouldn't quite have fit for a movie (e.g. "What happened then, well that's the play/and he wouldn't want us to give it away"). But for other songs - e.g. "God, That's Good" - they found ways to keep at least part of the song's lyrics, but just cut out the chorus or otherwise alter the song to make it fit for a chorus-less movie rather than a chorus-ful (so to speak) stage musical.
And in other movies based on stage musicals, switching up lyrics so that they're more suitable for the medium (and sometimes for much less apparent reasons, but that's beside the point) seems to be the norm. The first example I can think of is Rent, which went through quite a lot of minor changes in its script and lyrics when it was adapted for the screen. For example, part of Collins' solo introducing Angel in the song "Today 4 U" was changed from "Gentlemen, our benefactor on this Christmas Eve/whose charity is only matched by talent, I believe" in the stage version to "Gentlemen, our benefactor on this Christmas Day/whose charity is only matched by talent, I must say" in the movie. (There are quite a few other examples, but you get the idea.)
So, as a musical theater nerd, I can't help but wonder (even with my personal disappointment aside), why did Tim Burton and co. decide to completely cut the lyrics from such an iconic song as "The Ballad of Sweeney Todd" instead of just altering it? Was there a particular reason, or was it just a (relatively) "random" casualty of the song-slashing that always seems to happen when stage musicals are adapted for the screen? (Maybe the Ballad isn't as iconic as I always thought it to be?)