Explain this quote: "Good sir, why do you start , and seem to fear/ Things that do sound so fair?" What kind of literary device is it?
In simple terms, what is being asked of Macbeth here is: "Why are you so jumpy, as if you're afraid of things that actually sound great?"
Of course, Shakespeare's phrasing is much more elegant! The literary and rhetorical device he is using to create effect in this quotation is antithesis. Antithesis is a device whereby two contrasting ideas are set together in a parallel sentence structure, which we also see elsewhere in the play from the Witches—"Fair is foul, and foul is fair." Here, the phrases "seem to fear" and "sound so fair" parallel each other, but the "fear" contrasts with the "fair" to underline the strangeness of Macbeth's reaction, while also echoing the Witches' earlier lines. This helps draw the audience into the play, as it reminds them of the full context.
Meanwhile, devices such as assonance ("seem to fear") also help to give this phrase a poetic, chant-like quality, again similar to the Witches' chant.
The literary device used is alliteration (repetition of initial consanant sounds) with the repeated "s" sounds: sir, start, seem, sound, so. The next step is to consider what symbolic meaning (if any) could be conveyed, and in this case, one would think of a snake, or serpent, in that they make that sound. This fits with the context of the play, in that there is evil all around as the weird sisters give their prophecies and the seed of Macbeth's demise is planted.
This was what Banquo said to Macbeth, on observing his startled reaction to the Witches' prophecies that were made on the deserted heath, after the 'hurlyburly' was done, the battle was won.
When the hurlyburly's done,
When the battle's lost and won. 1.1
The Witches' have prophesied Macbeth to become the Thane of Cowdor and the 'King hereafter': those were two promotions at the same time! These fortunate and good promotions were what Banquo said sounded 'so fair'. '[W]hy do you start; and seem to fear' is a reference to his worried, anxiety-ridden expression, his thoughtful face, which their words have given rise to because Macbeth's ambitious nature has always dreamt of becoming the King of Scotland, but having the agents of devil tell him that it was an actual possibility, was what shocked him beyond reality.
Banquo: The instruments of darkness tell us truths,
Win us with honest trifles, to betray's
In deepest consequence. 1.3