The words of the witches are dominant in Macbeth, they stand out from the rest of the characters. How does this influence Macbeth's downfall?
I agree, the words of the witches are dominant. They influence Macbeth's downfall by planting the seed of ambition in Macbeth's mind and by luring him further with a false sense of security. There is no evidence of any real ambition in Macbeth before he hears the witches' prophesy. (This is probably because it happens so very early in the play.) What is evident, though, is how quickly Macbeth jumps to the thought of killing King Duncan after the prophesy is proclaimed. In fact, in the exact same scene, Macbeth already proclaims this:
My thought, whose murder yet is but fantastical / Shakes so my single state of man that function / Is smothered in surmise, and nothing is / But what is not. (1.3.139-142)
Two little pages and Macbeth is already thinking about murder! Ah, but there is fear in his voice. There is only one villain who is able to negate that fear enough to allow Macbeth to follow through with his evil deed, and her name is Lady Macbeth. It is almost as if Macbeth was just waiting for an opportunity to enter the realm of "vaulting ambition," which has always been known as Macbeth's tragic flaw: the main reason for his downfall.
Further, the witches give Macbeth false hope with their future apparitions. Macbeth cannot conceive how someone could be "none of woman born" or how Birnam Wood could come to Dunsinane or how Banquo could possibly have so many descendants (with Macbeth ordering their murder as well). Therefore, a false sense of security progresses Macbeth even further towards his tragic downfall.