But David/Trotwood is drunk, and at the theater, and doesn't know who or where he is after a while. Before getting there, he (the narrator) had been describing the dinner and the drunkenness and the state of the wine bottles in the usual first-person past-tense, but after a while these pronouns shift into a hazy "someone" (like "someone was vomiting in a corner"), and then finally says, "someone was me." But he goes on describing himself like this, from the outside, like Steerforth's vaguely unpleasant friend "a man." All of which works incredibly well when they're at the theater, and D/T is behaving very drunk in the box during the intermission (and after the intermission, although he doesn't seem to realize it and doesn't understand why everyone keeps shushing him), and the "someone" becomes frighteningly distant—especially frightening because his foster-sister Agnes is there in the box, too, and seems embarrassed to recognize him, as though he really IS someone else.
Later D/T has a melodramatic and very self-pitying hangover, and is ashamed about this transgression of personality. The "someone" has already accomplished the othering before D/T realizes it, though—we were getting it from the first "someone" (it is almost too obvious when the narrator identifies D/T as the someone), and so we're wrapped up in the little moebius-strip of D/T's/the narrator's understanding of himself, and when this understanding takes place.
