Final answer:
Macbeth addresses King Duncan with humility, expressing that fulfilling his duties is its own reward and does not require additional compensation. His seemingly humble exterior contrasts with his internal ambition, as revealed in his asides.
Step-by-step explanation:
In William Shakespeare's Macbeth, Macbeth addresses King Duncan with great respect and humility, acknowledging that any service he provides is its own reward. Macbeth's response to the king's gratitude - "The service and the loyalty I owe, In doing it, pays itself" - implies that fulfilling his duties to the king is an honor that does not require additional payment or thanks; his service to the throne is both a duty and a privilege that he willingly undertakes. This display of loyalty may be driven by Macbeth's desire to appear humble and devoted, especially in light of the recent prophecies that suggest his future rise to power. But, at the same time, his aside expressions reveal a more ambitious and conflicted mindset, with lines like "The Prince of Cumberland! that is a step On which I must fall down, or else o'erleap" hinting at his growing ambition and the turmoil within.