Banquo only mentions fear once in Act 2. This occurs in Scene 3, after the murder of King Duncan. Banquo tells the assembled group:
Fears and scruples shake us.
In the great hand of God I stand, and thence
Against the undivulged pretense I fight
Of treasonous malice.
By "scruples" he means suspicions. By "undivulged pretense" he is referring to the motivation and intentions of the as yet unknown perpetrator of the assassination. "Pretense" means "purpose" or "intention." Apparently he takes it for granted that the King's two attendants were only acting for someone else, since they had nothing personal to gain by killing the King. Banquo may already suspect that Macbeth is the real villain.
What Banquo is afraid of is that the person or persons behind the King's murder intend to commit more murders. He expresses his assurance that God will protect him and that with God behind him he is determined to investigate the crime and fight the culprit.
While standing guard with Fleance, Banquo is thinking about the prophesy that was delivered to Macbeth in Act I, sc. 3.FIRST WITCH:All hail, Macbeth! hail to thee, Thane of Glamis!(50) SECOND WITCH:All hail, Macbeth! hail to thee, Thane of
Cawdor! THIRD WITCH:All hail, Macbeth, that shalt be King hereafter!
Macbeth is already Thane of Glamis, but the current Thane of Cawdor and King are still alive. Banquo and Macbeth initially dismiss the prophesy as rediculous, but when news arrive that the King has named Macbeth Thane of Cawdor,
Banquo is concerned about the witches and their evil nature, thinking that just because the promotion is good for Macbeth, bad things may also happen.
"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"
Macbeth tells Banquo not worry and that they shall discuss it later. When we finally see Banquo again at the beginning of Act 2, He is still conserned about the witches prophesy, but is also concerned that his friend Macbeth still hasn't spoken with him about the witches. Banquo is so worried that he had a dream about the event,
I dreamt last night of the three weird sisters:
To you they have show'd some truth.(25) "
Although it is not directly stated, the implication is that if the witches prophesy continues to be true, then something terrible will happen to King Duncan so that Macbeth can become king.