Look around your room. Or, take a look at any jewelry you're wearing. Or maybe it's not jewelry: maybe it's something that you just carry around with you like a certain keychain or a ribbon you shove into your back pocket.
Then ask yourself: why?
My personal "charm," as I'm going to call it, is a necklace: the charm is a small circle with a swirl laid on top, and beneath it the word "karma." I've worn it nearly every day since I got it, which is bordering on 3-ish years, now. When I ask myself why I wear it, I really have no answer. The only answer I have is that it suits me; it's honestly really difficult for me to wear anything else, including a necklace that has mucch more sentimental value (a gold chain with a camel charm that my dad bought for me from Egypt when I was a baby; he and my mom gave it to me for my sixteenth birthday).
Why am I detailing to ya'll a portion of my jewelry collection?
Everybody has that one thing they take with them everywhere. Maybe not even everywhere, but it remains in sight in their bedroom or house, something like a charm that, for some reason, they're not too sure they could live without. It could be something that reminds them to act or think a certain way, or it could be a gift/remnant from a family member or friend that they keep around in order to feel attached to them.
The truth is, the reasons people carry things with them are as unique as they are. And charms and trinkets like these can carry some powerful memories and motivation. Some people feel completely lost without their "object of power," if I may so call it, and because of this they provide an excellent detail you can use toward characterization.
So I want you to think about it: do your characters have anything that they carry with them whenever they go out? If the answer's yes, then explore it. Try to really dig in to your character's psyche and figure out what makes that thing so damn important to them.
For example, one of my writing buddies has a character who carries around a coin from a currency no longer in use. Another one has a character who receives an item from a friend/love interest in order to make a ruse work, and then she never gives it back. Personally, I've once had a character who wore her father's military dog tags, and another who always wore a bow in her hair because it was the apocalypse and wearing a bow was the way she fought back against becoming such a hardened tough-ass (and earned her the nickname of "Bow," which she hates but it still makes me snicker).
More often than not, these kinds of trinkets serve as a reminder of a turning point, or a point of no return of some kind. They keep the character grounded, reminding them of what used to be instead of constantly forcing them to look forward. What's interesting is that this means the trinket can serve as both a positive or a negative thing: negative in that, if they keep thinking of the past or what that charm represents, they might refuse change, or push too hard against the inevitable, which makes them a stubborn mule.
Which makes for great characterization.
**Special note: your character doesn't have to have a charm. It's not a requirement. Heck, no matter how hard I think about it there isn't a single charm or trinket that either of my main characters in THE HOLLOW MEN have. It doesn't mean that they'll never have one (there's still two books that follow), but the fact that they don't have anything isn't a death sentence.
So don't freak out about that.
Mundane objects in novels. @Rae_Slater talks trinkets and charms, and how they create great characterization (Click to Tweet)