SCENE IV. The platform.
Enter HAMLET, HORATIO, and MARCELLUS
The air bites shrewdly; it is very cold.
It is a nipping and an eager air.
What hour now?
I think it lacks of twelve.
No, it is struck.
Indeed? I heard it not: then it draws near the season
Wherein the spirit held his wont to walk.
A flourish of trumpets, and ordnance shot off, within
What does this mean, my lord?
The king doth wake to-night and takes his rouse,
Keeps wassail, and the swaggering up-spring reels;
And, as he drains his draughts of Rhenish down,
The kettle-drum and trumpet thus bray out
The triumph of his pledge.
Is it a custom?
Ay, marry, is't:
But to my mind, though I am native here
And to the manner born, it is a custom
More honour'd in the breach than the observance.
This heavy-headed revel east and west
Makes us traduced and tax'd of other nations:
They clepe us drunkards, and with swinish phrase
Soil our addition; and indeed it takes
From our achievements, though perform'd at height,
The pith and marrow of our attribute.
There is always a good deal to say about doubling of roles in Shakespeare's plays. It's reasonable to assume that The Lord Chamberlain's Men (and, as they were later known, The King's Men) had no problem with doubling up on roles so that actors were kept busy and utilised in performance. Most of the current Arden editions feature a casting/doubling table as an appendix, and these can be helpful in terms of tracking how performers might be cast. Our first instance of it is Bernardo, who doesn't appear in this scene (Act 1 Scene 4). The actor might return soon enough as Reynaldo, Polonius' steward. The same actor, and the performer playing Marcellus, could likewise reappear as the various messengers who appear in the later acts of the play.
Hamlet's distaste for Claudius' drunken revels is an unusually negative response to alcohol in Shakespeare. Falstaff is, of course, the Bard's most celebrated boozer, and there's a case to be made for substantial alcoholism in Macbeth's Scotland. Hamlet's negativity towards it is interesting - scholars who enjoy the hunt for biographical clues in the plays might suggest that it's a reflection of Shakespeare's own views, but I find this unlikely.
The word wassail comes from Old English was hál, related to the Anglo-Saxon greeting wes þú hál , meaning "be you hale"—i.e., "be healthful" or "be healthy". It grew to be associated particularly with Christmas - wassailing and mumming were integral parts of the Medieval Christmas celebration. It's easy enough to trace the phrase back towards Old Norse, and Scandinavia in general, and imagine Shakespeare including it as an appropriate reference to Danish drinking.