In Act I, Scene 3, Macbeth and Banquo meet the witches on the field of battle. The witches greet the two men with prophecies about their (the men's) futures. They prophesy that Macbeth will be king and they address him as the Thane of Cawdor, which he is not.
When Macbeth finds out that the Thane of Cawdor has been executed and that he, Macbeth, is now being given that title. He starts to think about killing King Duncan to become king himself. At that point, he says
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs,
Against the use of nature?
This means, to me, that he is not usuall afraid of anything but that thinking of doing this does scare him.
I will have something to say about the idea of fear and its relation to Macbeth's character. Fearlessness is associated with the virtue of a true soldier, the light in which Macbeth is presented in the initial scenes of the play. But like sleep, fear too is a part of the inevitable workings of nature on the human mind and thus to overcome fear completely is also to overcome the bounds of being human altogether. That is how we see Macbeth, the tyrant towards the end of the play. He is reported to hang one and all who gets afraid of anything.
Macbeth, as seen through the eyes of Macduff, is a fierce autocrat who wants to rule by creating his fear in the mind of the subjects and yet cannot tolerate someone under the spell of fear.
Macbeth was all fear before and at the time of the murder of Duncan. One may remember his frights about the phantom-dagger, with a voice, he heard in the corridor and all his anxieties about the blood stains on his "hangman's hands" and so on in the murder scene. But after Duncan's murder it is Lady Macbeth who becomes a representation of fear, something that culminates in her final scattered words while in a state of sleep-walking.