"During the filming of the Lord of the Rings, Christopher Lee corrected Peter Jackson on the sound of being stabbed in the back.
From the DVD commentary:
"When I was shooting the stabbing shot with Christopher, as a director would I was explaining to him what he should do"... "And he says, 'Peter, have you ever heard the sound a man makes when he’s stabbed in the back?' And I said, 'Um, no.' And he says 'Well, I have, and I know what to do.'""
He had been involved in some super secret operations for the British during WWII, which is presumably where he knew that from."
https://www.imdb.com/name/nm0778734/