There is no singular discourse that unites all of the characters of the play: rather the witches, Macbeth, and Lady Macbeth all share in a particular way of rendering language which begins with the witches' incantation at the beginning of the text and follows through to end of the play. Macbeth receives their language, passes it on to Lady Macbeth in the form of a letter, who then reconfigures it in a persuasive manner to lure Macbeth to kill. The seductive notion that their prophesies can be 'true' causes Macbeth to believe the witches, to trust Lady Macbeth's words, and his character is literally eaten alive and possessed by their words until he is a shell of a man. Banquo, in contrast, merely hears the witches' language, but never adopts their method of speech, and no character 'mirrors' his words as Lady Macbeth's does Macbeth's nor affects...

His inability to assume the witches' language means his actions are unaffected by their words throughout the play.
Macbeth's free will in obeying the witches is only questionable in the sense that our ability to acquire language is an act of free will: the acquisition of language is natural and inevitable, but how we interpret that language and choose to use it does involve some human volition. Through a Lacanian understanding of the play, it is possible to see Macbeth's crime both as chosen consciously, as he chooses to accept the witches' language to characterize the future, even though once he enters into their discursive framework, it becomes increasingly impossible for him to see the world outside of its sing-song terms of violence, destruction, and false promises of safety.