In Act III the relationship between Banquo and Macbeth has deteriorated since their encounter with the witches in Act I.
Banquo has demonstrated that he is a cautious man. After the witches have made their prophecies in Act I, Scene 3, Macbeth is persuaded by the appearance of truth in these predictions, noting that as the witches have said, he has become the Thane of Glamis and the Thane of Cawdor. Since these things have come to pass as predicted, Macbeth asks Banquo if he does not hope that the prediction that his sons will be kings will also come true. Not so easily swayed, Banquo cautions Macbeth against believing so quickly the words of the witches:
And, oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence....(1.3.132-135)
Further, in Act II, Scene 1, Banquo's actions appear to reflect some anxiety. As he walks the halls of the castle at Inverness with his son Fleance, Banquo has cause to be nervous because he knows that Macbeth has readily believed in the witches' prediction about his becoming king without questioning the problematic nature of their other prediction of Banquo's future as the father of kings.
On the night of their arrival, Banquo asks his son Fleance if the moon is down and when Fleance tells him that it is, Banquo hands his son the sword he has been carrying, but as soon as he hears someone coming Banquo immediately takes it back. The person Banquo hears is Macbeth, who is still walking around even though the guest of honor, King Duncan, is "a-bed."
It is not long before Macbeth is confronted by Banquo holding his sword, asking "Who goes there?" Macbeth replies, "A friend" (2.1.10-11). Banquo then gives Macbeth a diamond necklace that the king has asked him to carry to Lady Macbeth as a gift for her hospitality. Banquo also tells his host that he dreamed the previous night of the "three weird sisters," adding, "To you they have showed some truth" (2.1.20). To this, Macbeth responds:
I think not of them.
Yet, when we can entreat an hour to serve,
We would spend it in some words upon that business,
If you would grant the time (2.1.22-25).
Here Macbeth suggests that he wishes to talk more with Banquo. Continuing, he says that if Banquo remains with him, there will be some reward for Banquo. Banquo replies that he will comply with Macbeth as long as nothing goes against his conscience:
MACBETH: If you shall cleave to my consent, when ’tis,
It shall make honor for you.
BANQUO: So I lose none
In seeking to augment it, but still keep
My bosom franchised and allegiance clear,
I shall be counselled. (2.1.24-29)
As a result of this conversation, Banquo realizes that Macbeth intends to be king. Also, if Macbeth kills King Duncan in order to achieve this goal, then Banquo could be in danger, especially if he does not, as Macbeth has said, "cleave to [his] consent." (i.e. Remain loyal to Macbeth when the time comes).
In Act III, Scene 1, Banquo speaks in a soliloquy. He acknowledges that Macbeth is king as the witches have predicted, but he fears that "Thou [Macbeth] play'dst most foully for 't" (3.1.2-3). Since he suspects that Macbeth has engaged in a foul deed in order to become king, Banquo wonders about how the witches' prediction about him may come to be. For one thing, he realizes that he would have to be dead in order for his sons to become kings. Consequently, Banquo loses his trust in Macbeth, especially in light of their conversation in Act II. Their relationship has now deteriorated to one that involves distrust and possible enmity rather than friendship.