Does watching the opera in the Fifth Element count?
Eric Serra is one of my favorite film composers. He had never worked with an opera singer before this movie, and when he wrote the Plavalugna scene he thought nearly half of it would be technically impossible for a person to actually sing, that they'd have to pull off much of the piece via trickery. On the day, the singer (an eastern European woman, as I recall) showed up and as she was taking her coat off, getting a drink, etc, she was just sort of casually humming the full piece from her memory of the sheet music. Serra was like, "Whoa, wait a sec....", and she was all "Right, let's get to work", at which point she promptly sang the whole f**king thing live. They ended up tweaking a very few bits here and there - things like smoothing out breaths, or flattening a couple transitions - with the goal of making it sound a bit more 'alien'. But nearly the whole thing as it is heard in the final is just that woman singing.
25 years after that movie came out, I still sort of can't believe it.
