Okay I've decided I'm going to ramble about the Nichols relationship dynamics I've thought about so yap down below
Okay so in my head Nichols has been part of the Obra Dinn crew since her first voyage in 1796 and was hired as 3rd mate. Hoscut was already 1st mate, Martin and Davies were later hires. It would've been one of Nichols' earliest voyages since graduating from a midshipman, but not the very first. So to me he'd be early 20s.
Because he's so young Hoscut acts sort as a mentor figure and the two are initially quite close, but then Nichols gets older and becomes more bitter and cynical and they grow apart... Nichols strikes me as the sort to care a lot about how he appears to others so he would try to appear friendly on the outside while becoming more and more resentful within. It would flare out from time to time but usually only at people he sees as beneath him - it doesn't matter what they think of him. Overall though, I think he'd want to have a good reputation. Anyway I got off topic. Hoscut would do things that would frustrate him and Nichols wouldn't really say anything because ultimately he would always want to suck up to a superior officer. Hoscut would probably much prefer to just talk things out but Nichols would never want to even slightly risk damaging his relationship with Hoscut. Even if this is what actually makes it worse.
As for galligan there are a few different ideas I like to think about. There's the default settings of Galligan has been Nichols' steward since he became a mate which works fine but also. Because of Galligan's age I think it could be interesting if he was a servant that worked for Nichols' family that just got taken with him when he became a sailor... In this scenario Galligan would probably have known Nichols for most of his life. There's also the option of Nichols burning through stewards because he doesn't treat them well so they keep quitting the first chance they get and Galligan is just the first that really clicks. I think they're both terrible and Galligan knows that he can get away with more by using Nichols, and Nichols likes having someone that's like him around that he can rely on to help him get away with things.
It's definitely not a 1:1 by any means but I think of them like iago and othello sometimes... Galligan subtly pushing Nichols towards doing bad things... Not that I think Nichols isn't a bad person capable of doing bad things on his own I just don't think he's very good at coming up with the ideas. Galligan thinks of schemes and then makes Nichols think they were his idea the whole time. Then if Nichols makes a mess of things (which it seems like he probably often does given his tendency to panic and be useless) Galligan helps clean it up. Though I think if things went badly enough he'd throw Nichols under the bus to save himself.
I need to think more about Nichols and the captain... I think witterel would trust nichols on the basis that hoscut does and he trusts his judgement. I want to write a fic that happens between the murder and the calling where it's witterel finding out what Nichols has done and he thinks back on their relationship and what signs he must have missed... that would be fun
As for Davies. I think of him as a recent hire. And so he finds Nichols really offputting. Everyone else has known him for so long and I imagine him as slowly getting worse so they don't really notice. But then Davies meets him and is like "what is wrong with this guy and why does no-one else seem to notice." He never shows any sign of disliking him or anything but he does think he's odd...
