Christopher Lee was to play Salem...
But wasn't available.
As much as I love Christopher, I don't think he would have been as good as Max.
There's no way of knowing that now. But judging by their facial features, Lee is definitely a "saner" one ) At first Von Sydow seemed to me not crazy enough for the part, though I was amazed by his flexibility in this movie, and that's just 2 years before the Exorcist!
have a nice life!