I think it's gonna be something you'll have to pay close attention to if it doesn't come naturally.
When I write closed-off characters, I try to put myself in their shoes. I take into account their background, which shaped their personality, and why they wouldn't be open to others.
It also depends on what kind of characters they interact with. Maybe they're more prone to gravitate toward certain personalities, and be less spikey when talking to someone they find are a lot like them.
Usually, people who aren't trusting of others don't talk a lot. They mostly keep to themselves, maybe even find it suspicious when someone comes up to them and tries to strike up a conversation.
I guess you would have to study why someone acts a certain way. You can be friendly and polite to someone without trusting them. Or you could be silent and avoidant- being more isolated.
Think, if someone you didn't like came up to you and started interacting with you, how would you react? Sometimes you have to pull from your own experiences to write something for your characters if you want to make them believable. Think like your character and stick to that, it's kind of hard because sometimes you just want one character to have a relationship with another right away, but it takes time for trust to develop if that's the goal.
If you have any questions about what I said feel free to ask, I'd be happy to clarify. If that didn't help at all then I apologize, sometimes it's hard to put thoughts down and make them sound coherent.
