As an SOE member, he was more than likely the one stabbing.
“When Wormtongue rises up and comes up behind Saruman to stab him, of course it was my job as director to talk to Christopher Lee and to explain to him what I wanted, so I started to go into this long explanation about what sort of sound he should make when he got stabbed,” he explained.
Lee: “I seem to recall that I did say to Peter, ‘Have you any idea what kind of noise happens when somebody’s stabbed in the back? Because I do.”
Jackson: “He proceeded to talk about some sort of very clandestine part of World War II. He seemed to have expert knowledge of exactly the sort of noise that they make and so I just sort of didn’t push the subject any further.”