Ian McKellen died filming this scene. That's why he's not in the rest of the movie. But they'd already filmed him in the next 2 movies, so they CGI'd his clothes white and said he came back to life.
The balrog has actually sued for residuals and has recently joined the screen actors guild. (Although the manslaughter case in nz is causing some issues in his career)
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.'
Ian knew the chaos would cause massive amounts of traffic from fleeing locals, so when his fellow actor suggested driving back to the city until things calmed down, he berated them and said to take his private jet back to the states.