(Macbeth to Banquo) Echos the first words said by the witches. It is a fair day because they have one, but it is a foul day due to weather and all the deaths. It is more doublespeak.
(Banquo to Witches) Banquo wants his fortune told too. He has disdain for the witches but wants to know. He is a good foil for Macbeth. When Macbeth gets a good prediction, he becomes ruthless. Banquo stays loyal and dies.
If you can look into the seeds of time, and say which grain will grow and which will not, speak then to me.
(Banquo to Macbeth) Banquo is developing suspicion of the witches' fortunes and thinks perhaps they should not be heeded. The witches are classified as dark beings, not just supernatural ones.
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 us in deepest consequence.
(Duncan to Malcolm) It is impossible to tell a person's true thoughts. There is no way to tell if somebody is merely a good actor. This is ironic since the Thane of Cawdor and Macbeth both managed to fool Duncan.
There's no art to find the mind's construction in the face.