One of the best Sweeney's I have ever seen wasn't able to sing a lot of the score and he was still brilliant. He incorporated it into Sweeney's frustration and madness. He was flat for half the show but for some reason it didn't really matter to me that much. This scenario reminds me of Depp, an actor whom I admire.
Depp's singing isn't worrying me. I've come to accept the fact that he won't be able to sing the score like George Hearn or even Len Cariou. If his acting is perfect, and I have every reason to believe it will be, than he'll already be a good Sweeney in my eyes. In addition to the difficult score it is one of the richest and most difficult acting roles in all of musical theatre, and I think we often tend to forget that. Though the music is crucial, there's a lot more to the role and I think Depp will remind us of that.
"Sing the words, Patti!!!!" Stephen Sondheim to Patti LuPone.