Christopher Lee was allegedly there to give advice to make the fight more realistic.
'Have you ever delved into the infernal pits of despair, Peter? Have you ever met a creature made of pure evil? Have you seen your comrades fall, taken one by one by a Balrog, until you're the last one standing? I was in the SAS during the war, Peter, and you can't imagine the atrocities I have suffered and given in return. Do you know what sound a Balrog makes while he is hurled across abysses of torment, Peter? It's a sound that makes terror feel like pleasure. And yet I crave it, I cannot forget it and I long to inflict such a pain to another Balrog, for I loathe them, and I know no mercy for them and I have lost my soul. Oh, yes, Peter.'