When the witches' predictions begin to influence Macbeth, what thoughts form in his mind?

The best answer to this question is found in Act I, Scene 3, in which Macbeth both hears the witches' prediction and discovers the first part of the prediction came true. After hearing that he has been unexpectedly named Thane of Cawdor, Macbeth says "Two truths are told, / As happy prologues to the swelling act, / Of the imperial theme" (127-29). In this aside, we can see that, since the first portion of the prophecy has come true, Macbeth is becoming increasingly interested in the latter portion of the prophecy, the portion that predicts his rise to the crown. More specifically, we can see that Macbeth's thoughts are turning increasingly to the prospect of being king, suggesting that it won't be long until he takes matters into his own hands and usurps the throne by violent means.