This post is my plea for every SBCL devotee to go back and (re)read the extra chapters. I believe Chapter 96.5 was the first. It was released during the GWA but takes place during BoC. In this chapter, Sebastian appears to be affected by whatever liquor Joker pours him and shares a personal anecdote about one of his earlier... shall we say, consequential decisions... in filling the role of butler.
I'll refrain from posting the whole chapter here, but this is where the real fun starts. Sebastian, disguised as Black, is offered a seat at the table, already including o!Ciel (obviously disguised as Smile), and Joker pours the former a drink.
I love this page in particular because Sebastian's first line is very deliberately phrased, making his second line all the more intriguing. The fact that, for the past year, he's never been officially employed as a chef but has been made to do the job of one, on top of many others, is a particular semantic detail that tells me he's still binding himself to the contract despite the fact that he's not actually speaking to Ciel. I believe Ciel is sitting across from him at the table, and the immediate cut to his reaction stresses the importance of his listening ear. If his presence at the table means that Sebastian is still bound by the contract here, then Sebastian must be telling the truth when he says that he, despite being a demon, has never experienced drunkenness.
This lends credence to the idea that Sebastian's past experiences with the human world, pre-contract-with-o!Ciel, have been extremely limited, possibly exclusively consisting of bloodlust and orgies and various forms of destruction/consumption, possibly to such an extent that Sebastian could hardly be considered a person prior to said contract.
Joker shares a harshly bittersweet story about Doll having believed in the tooth fairy back when they and the rest of the orphans were living on the streets. I cried re-reading it even though you know exactly how it's going to go. Sebastian raises the glass to his lips.
At this point, we see that Sebastian has stacked up five or six glasses with another in hand, but only if you really look at the bottom left corner of the first panel, where your eyes are automatically drawn to the next speech bubble in the opposite corner. Even the little sparkles in the first panel draw your attention away from that bottom left corner of the first panel, building the final comedic reveal.
Aaand this is where Sebastian begins to overshare and recount his experience pulling out one of Ciel's teeth without his permission during the early days of their contract, framing his master's horrified reaction to his sudden intrusion as something relatable-y unexpected, petulant, and amusing in its childishness, even though his audience reacts with the same horror. We know Sebastian is telling the truth when he says he thought he would be doing his master a favor, but for once, after what I counted to be 22 glasses, his devilishly earnest intentions fail at passing him for an eccentric human and betray his otherworldly nature.
All the extra chapters are either outlandish AUs or one-off missing scenes set in the past, the latter of which always involve at-least-slightly OOC nonsense for the sake of humor + silly costumes (save for this one, really). I choose to believe that the only OOC element present in this extra chapter is Sebastian taking a seat at the same table as his young master, even if only for the sake of his disguise. Everything else, I take at face value.
