In his biography of the director, A Life in Darkness and Light, Patrick McGilligan implies that Hitchcock planned to incorporate the following into The Blind Man (yet another unrealised project)...
The film would open in Covent Garden Opera House. A soprano, say Maria Callas, would be singing an aria and just as she hits the high note at the end, she'll notice, in once of the boxes, a man being stabbed. Her high note will turn into a shriek of horror, which also happens to be the highest note she's ever hit and it brings the house down with applause. The next shot will be of a dead body falling from the box... and that's as far as I've got!
— Alfred Hitchcock