Final answer:
Banquo cautions Macbeth to be wary of the witches' prophecies, hinting at the deceitful nature of such fortunes, which could lead to dangerous outcomes. Macbeth, however, is entranced by the idea of becoming king and ponders the role of fate in achieving his ambitions.
Step-by-step explanation:
In Macbeth, Banquo warns Macbeth about being cautious of his good fortune after becoming the Thane of Cawdor. Banquo suggests that the prophecies that led to Macbeth's rise, as provided by the witches, might be deceitful or lead to consequences. This warning is shown when Banquo reflects on the nature of the witches, comparing them to instruments of darkness that could be telling half-truths to win Macbeth's trust only to betray him later. The text: 'Banquo. That trusted home Might yet enkindle you unto the crown, Besides the thane of Cawdor. But 'tis strange:' indicates Banquo's cautious skepticism about the witches' prophecy and its implications.
Despite Banquo's warning, Macbeth contemplates the possibility of becoming king even if it means not taking any action himself, as shown in his aside: 'Macbeth. [Aside] If chance will have me king, why, chance may crown me, Without my stir.' Ultimately, this leads him on a path of ambition and destruction.