So, I decide to start another story, this time writing about a 12 year old moving into a small town, and I want to explore the theme around community, belonging and facing the future. I was wondering, should I develop the characters first, or do I start the plot then the characters?
There's really no definitive answer; some people work one way, some work the other, and some change from project to project. I'd say to start with whatever is clearest to you right now, even if it isn't much. If you have a pretty good idea of what kind of story you want to tell, think about characters who could fit into it. If you have some characters in mind, think about what might happen as they interact and go about their lives. In the end, your characters will influence your plot and your plot will influence your characters—and together they'll end up telling the story.
It sounds like the community she's moving into is very important for this project. You might want to start by thinking about that, and especially how she fits into it and in what ways she isn't a good fit. Maybe rather than starting with one or the other, think of it more as gradually defining the community and her (and the other characters) and the ways in which they all interact. You don't want to fully define either the community or the characters first, since to such a large extent each defines the other. It's going to be a matter of going back and forth again and again and developing them alongside each other, partly in harmony and partly against each other. It seems to me that your story to a large extent will grow from that interaction.
Yeah, if you start writing it will gel as you go. I think you'll find it will be how the community affects your MC or vice versa.
I would choose characters first for that type of story. Give them needs that conflict with one another and a plot will happen. I wouldn't set it up as hero vs antagonist either. Everyone should be a selfish hero of their own world.
As previously stated, it is completely up to you. In my opinion though, if this were my story, I would start off with the setting for imagery. I like "show and not tell" so I would describe the landscape and lets say the car ride into the small town and then when it is finally revealed that the MC was moving it leaves the reader with a nice "ah-ha" moment.
I'd like to say "Characters" but I find that I start to like the characters too much and try to find ways to shoehorn them into stories and situations they have no business being in. I wind up with tortuous plots and stuck in stupid corners with no way out.
