The whole "acid jail" conversation in Harrow the Ninth is very funny (You mock Mercymorn? You mock her efforts like the art critic? Oh! Oh! Jail for emperor! Jail for one thousand years!), but I feel like it's also a good example of characterization via banter.
I think that a lot of complaints about modern comedic beats being too "quippy", too "well, that happened", Joss Whedon-esque, etc. are ultimately because when you have everyone fall into snappy banter with each other seamlessly, it can wind up feeling like every character has the same voice- a voice which coincidentally happens to sound exactly like the author's own sense of humor and maybe the groupchat they have with their two snarkiest friends- even when the characters are wildly different from the author and also each other.
But when you can make the characters banter with each other while also making every character's own voice and personality inform the dialogue, it can actually work really well, and I think Tamsyn Muir did a good job here.
"That's a muffin!" - Of course Augustine is the one to jump in and start shit with Mercymorn, who else? The book has made it clear that they've been sniping at each other for a myriad, so it makes total sense that he's the one to begin the whole conversational derailment.
Also note his choice of interpretation for this Rorschach test of a scribble: something frivolous and harmless when he knows full well Mercymorn is trying to represent a deeply disturbing monster. I don't think it's accidental that he picked something silly while needling the woman who takes everything seriously- he knows how to get under her skin.
"If you take that main squiggle for an eye." - Equally naturally is Ianthe immediately piling on. This girl was raised in a culture where "Regina George" is a respectable career path, and she is employee of the month or else. The fact that she's trying to get on Augustine's good side as part of her politicking would probably see her throw him a bone regardless, but she also just really enjoys bullying people- and she's good at it.
She's the only one to describe a specific feature of the drawing, I think because it puts more emphasis on the idea that Mercymorn drew badly as opposed to Ianthe herself just not interpreting it correctly. She wants her listener to understand that they messed up and she is making fun of them for it.
"I thought it was a flower." - This one is almost a little surprising at first glance. When did the Reverend Daughter of the Ninth ever even see a flower? They grow snow leeks and emotional instability on the Ninth and not much else. Are snow leeks flowering plants?
But there is something fitting about the girl in love with the sleeping shell of the Earth's own soul picking a living plant for her interpretation...
Beyond that, I think it's mostly Harrow seeing that they're not going to shut up and adding her honest, if unsolicited, two cents. She might not care about this conversation, but she's not going to be left out of a debate- even as thoroughly unmoored as she is, there's still the person who became almost immediate intellectual friendrivals with Palamedes Sextus in there.
"No, yes, I agree... there's something florescent about it." - This is so quintessentially John Gaius that it was the first thing I noticed and the entire reason I wrote this post.
Joining in the silly banter is very in keeping with the humble-despite-the-crown image he wants to project- An emperor you could grab a beer with!- but you'll notice that he doesn't actually offer his own opinion, he just backs up Harrow's.
(What would I know, I'm only God.)
It could, of course, also just be what he actually thinks- if we're going to call people out for being psychosexually obsessed with Gaia Alecto, well. He was here first.
But think about it, though.
He's joining in the same as the rest of them, but if Mercymorn gets mad he has that extra layer of plausible deniability- technically he didn't say it looked like that, she said it, he just agreed with her. And anyway, the poor girl could use a little support, Mercy, can you really be mad at him for trying to gently encourage her? (He does like Harrow best and he's not really trying very hard to pretend otherwise. There is no house in which Ianthe Tridentarius has ever been the favorite child.)
Getting to play around and play favorites while coming off as relatable and kindly, and always leaving himself an out in case he could be held responsible for literally anything, ever.
The Emperor Undying in a nutshell.
"Thought it was a snake in a bush." - Last but not least, an illustrative example of Gideon the First's lot in life (and death, repeatedly): joins in because if everyone else is doing something stupid, he might as well. His god has led, and thus he follows.
No one really expected him to be part of this conversation, but just because he's quiet doesn't mean he lacks any thoughts of his own, he is clearly thinking about it. After all, he isn't copying anyone else's answer, John.
Would never have shared those thoughts if the rest of them hadn't gotten distracted, though, because unlike his fellow immortal dipshits he's actually trying to pay attention to the briefing.
God forbid we get anything done around here.