Through Macbeth's with other characters, the audience is able to judge just how shaken Macbeth is. Shakespeare has already introduced the witches twice to the audience in scenes 1 and 3, so the audience is well aware that while the witches can move actions into motion, they cannot control people's fates. So, as they address Macbeth by three titles, the audience can tell that Macbeth is being set down a road they are creating for him.
As the witches greet them, Banqo notices that his partner is shaken and asks him
Good sir, why do you start, and seem to fear
Things that do sound so fair?
Once Macbeth learns that King Dunan has presented him with the title Thane of Cawdor, he begins to question the witches premonitions as well as his desire to become king. Banquo again cautions his friend to weary of the witches true desires.
That, trusted home(130)
Might yet enkindle you unto the crown,
Besides the Thane of Cawdor. But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's(135)
In deepest consequence—
Cousins, a word, I pray you
It is also through his words to his wife that we see his desire to be king. While she reads his words (his delineation of the events) we see that he wants to be king, but we also see that his wife realizes that he will not be strong enough to complete the tasks she feels he will need to be king.
Whiles I stood rapt in the wonder of it, came
missives from the King, who all-hailed me ‘Thane of
Cawdor’; by which title, before, these weird sisters saluted me, and referred me to the coming on of time with ‘Hail, King that shalt be!’ This have I thought good to deliver thee,
my dearest partner of greatness, that thou mightst not lose the dues of rejoicing, by being ignorant of what greatness is promised thee.