In Act 1 Scene 3 of the play 'Macbeth' by William Shakespeare, Banquo tries to warn Macbeth that the witches' predictions might lead to evil. Macbeth and Banquo are comrades in arms and are full of the jubilance of winning the battle together. Yet now the witches look set to divide them. In his warning, Banquo looks as if he is considering the possibility that the witches might be representations of evil, of the devil 'instruments of darkness.' His words do have some effect, as Macbeth seems to weigh up the implications - he appears to consider the moral consequences of the prophecy. However, upset though he is about committing the deed, he seems to keep this resolve hidden from Banquo,widening the division even more as he covers these thoughts up from his friend.
In Act I, Scene 3 of Macbeth, Banquo and Macbeth are just riding in from the battle field, leading their troops behind them. They dismount and are walking alone to see King Duncan after winning a successful battle against the traitorous Thane of Cawdor. On the way, Banquo and Macbeth come upon the three witches who have been preparing and waiting for Macbeth.
The witches, once they are bidden by Macbeth to break their silence and speak, give prophecies to Banquo and Macbeth. Banquo will be the father of kings and Macbeth will be Thane of Cawdor, who is at the moment held as a prisoner, and will be King. Macbeth is startled at the news.
Momentarily, riders from the King approach and tell Macbeth that the title of Thane of Cawdor has been bestowed on him because the traitor will be executed. He then asks Banquo if he believes the witches now because of the deliverance of their Cawdor prophecy. This is where Banquo administers his warning, saying that sometimes evil works by trickery. He means that evil may tell truths and grant trifles in order to lure the innocent into great consequences of wrong and evil acts in order to have the greater promise that they now lust for. Banquo spoke a true warning to Macbeth as events later reveal.
That trusted home
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
In deepest consequence. (Act I, Scene 3, Lines 120-126)
Throughout the Scene 3 of Act 1, from their very first utterance, Banquo seems to be leery of the witches and the news they bring Macbeth and himself. In Line 45, he seems to question and doubt their actual physical appearance:
You should women, And yet by your beads forbid me to interpret That you are so.
Banquo is not impressed by or afraid of the witches which makes him an objective and appropriate adviser to Macbeth. His suspicion of them seems to increase as he wonders in Lines 83-4:
Were such things here as we do speak about? Or have we eaten on insane root That takes the reason prisoner?
Banquo's suspicions and premonitions about the witches seem to grow until until they culminate in his outright advice to Macbeth in Lines 124-7:
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.
The lines signify that ultimately, there may be more harm than good even though the half truths by "The instruments of darkness" may seem attractive for the moment.