From Peter Jackson’s 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.’”
The crew said that they knew Christopher Lee had been in the British Royal Air Force Intelligence Service in World War Two, and they didn’t really push him for more information about how he knew in such detail exactly what noise a person makes when this is done to them.
He wouldn’t have told them anyway.
When pressed by an eager interviewer on his SAS past, he leaned forward and whispered: “Can you keep a secret?”
“Yes!” the interviewer replied, breathless with excitement.
“So can I.” replied a smiling Lee, sitting back in his chair.