Overseer of the Duckpond

You've described him as inscrutable and I think in many ways that's on the money. My headline on John is that he is by nature a quiet man, who cares deeply for those few he loves and has an incredible understanding of human behaviour (other humans' behaviour, at least), but this is wrapped in a thick shell made of being-a-british-man-of-a-certain-age; my usual red line when writing John is that he will not be expressing a feeling to anyone except Sarah (and perhaps not even noticing he has one) until someone is bleeding and/or it's been bothering him for at least half an episode.

This is not a rare affliction in Causton CID, exactly, but it is an important facet of John, especially as concerns his relationships with his sergeants, which are usually what I'm focused on when I'm writing him. With them on the whole I'd say he attempts to maintain a professional relationship, leaving most other things unsaid and making a doomed effort to retain plausible deniability as to how much he loves them, despite his actual behaviour often standing in contradiction.

Some of my favourite John bits:

- When he uses his sergeants' first names! He does this while talking to them unprompted exactly twice, once while talking to Jones in Murder of Innocence, and once to Winter in Death of the Small Coppers (we were robbed of ever getting a Nelson moment like that and I still mourn). I love a character-who-only-surnames-uses-firstname moment, so those are Big for me, and I especially enjoy how these two instances compare and contrast with one another. I think they both fulfil the purpose of trying to reach the sergeant concerned. Metaphorically, in Jones' case - iirc Jones is refusing any sort of police protection despite an attempt having just been made on his life. Barbaby firstnaming him feels more calculated, perhaps an effort to get through to him, or strip away the protective, professional veneer of "DS Jones", to remind Jones that he could as easily be a victim as anyone else. In Winter's case, meanwhile, Barnaby does not have time to consider anything. His voice is the only thing that is going to reach Winter before that arrow, though he tries to run to him anyway. I don't think he means to use his first name in that moment - it's a raw instant of exposure, betraying his care for Winter, and naturally they will never, ever talk about it. :)

- The scene in The Christmas Haunting, when he tells Nelson that a murder wasn't his fault. It's kind, and I love it so much - the way he notices Nelson's upset (not that Nelson's being subtle about it), and takes the time to reassure this poor guy who's been a DS all of five minutes that he'd done nothing wrong. Also, for contrast, see him telling Jones in Dark Secrets that a murder isn't about him. The situations aren't identical (Jones has the tone of a man asking for an excuse while Nelson has already decided he's to blame, Nelson has far less experience, etc), but perhaps this shows mellowing over time, or that his eventual partnership with Jones has led him to be kinder with his sergeants. We also have the conversation in The Lions of Causton, when Winter says he's never lost one (a killer, presumably) before, and Barnaby just tells him quietly that the killer had made up his mind - and for me there's just so much going unsaid there, but Barnaby is meeting Winter where he's at and reassuring him, just as he tried to Nelson, that there was nothing they could have done.

- During Winter and Nelson's first episodes, he makes sure they're introduced to Sarah - even ignoring the knock at the door from Winter so she'll go to answer it - like he wants to check them with her. In my gentle headcanons this is from an unspoken understanding that his sergeant is their sergeant.

- In The Witches of Angel's Rise, when they arrive at the psychic fair and he has the most horrendously awkward check-in with recently-bereaved Winter as to whether he's okay to go in there. Yes, Sarah had reminded him, told him to look out for Winter, so we had the "are you sure you should be in today" earlier in the episode, but the later one - Barnaby is recognising that Winter is in a potentially vulnerable stare, that the psychics/mediums might be upsetting to him (or might even prey on him), and checks in again (eventually). Also later in the episode he stops a guy hitting Winter with a stick. Big fan of that.

- In Let Us Prey, when he's afraid that his child won't like him. It's so vulnerable and sweet. We've seen in other episodes that he's not hugely social, he doesn't seem to have many of his own friends, and he's content with that, right up until there's this one person that it matters more than anything else to him that they like him - but he doesn't know how. Just leave me here in a puddle of my own tears I suppose.

Probably have to leave it here or I'll never stop but yeah. Hope this has been at all helpful? And good luck with your writing!