Today I'm feeling a little extra single, so I'm just gonna go through a list of different ways to show trust or intimacy between two characters (or real life ppl, I mean u never know).
Hands. Omg hands. Mostly hands touching each other. This can be a person's own hands touching themselves to establish a level of comfort in their body, or with someone else showing deeply intimate feelings. If it's their palms touching it could be them connecting their energies (like when they make their pairing official), if it's finger tips it could show a trust big enough for experimentation and so on. Hands communicate a lot, but especially intimacy so please use hands in your writing.
Eye-contact. It can be a show of a characters general kindness and confidence if they, instead of nodding at people when saying hi on the street, just keep eye contact and smile. Eye-contact can be used in an alluring and flirtatious way. You could write whole paragraphs of your MC trying to decipher another person's Eye-contact. It also upgrades every smutty chapter by 100% because it makes the characters confront each other and brings them back to reality. It makes a scene much more real. It can also be used to establish a characters confidence - where are they looking? Are they thinking abt where they are looking? What does this mean?
Communication. This may just be me longing for a relationship where I don't have to be the mature one, but communicating is so fucking sexy. Literally, communication is the best way to show trust between your characters, and it will probably influence your readers in a positive way if you sneak it in there.
Ears, for some reason. Once again, this is related to communication and listening to other people. If they notice another person's ears while they're talking, it could show-case listening. Also, noticing little things about people in general is rally intimate and can be used to share that, especially if it's not a very observant person.
Sharing physical things with emotional importance to the character. Fx. letting someone/a character borrow ur MC's favorite book or Lipgloss, and the character/person makes sure it gets back in good condition. This will show that they trust and/or care for the other person and respect their interests. As someone(a Taurus) who gets very possessive over my favorite things, this is definitely a huge one for me, so don't be fooled by how small/general it is.
If it's parental love/intimacy/trust, I find that arguments can be a good show for it as well. A safe environment is also one where you/a character is allowed to lash out when their feelings get too big to handle. It shows maturity and patience from both sides if an argument can happen, and both learn from it constructively.
Discussing art. Literally such a cute trope I love it deeply. Especially when the two characters disagree, and while it isn't mentioned or made too obvious, it's still clear that they obviously respect the other person's opinion. It is, however, a very nice playground for flirting, like with every other fight (enemies to lovers fans have a point).
Apologizing. Need I say more? No, but I will. Apologizing shows an ability to take accountability for ones actions, which is needed when showing trust and intimacy. Even if it's just a small "I'm sorry" after accidentally hitting someone on the nose. Don't overuse it, though, cause that might make the character seem insecure (which could be intentional, but just showing through apologizing gets boring and annoying over time).
Have them stand/lay next to each other unintentionally and without them noticing until a little late in the moment. Like if they get new seats in class, and one doesn't notice until 20 minutes into the first class. When they do notice, though, they might start panicking a little bc "omg they're r i g h t t h e r e" and then they make a move or smth (idk it's ur story).
Exchanging annotated books. It's like a peak in the other persons/characters brain. Besides, books are incredibly intimate and the favorites will typically be ones that a person relates to in some way, shape or form, making just that a simple peak at their emotions. Books about bookworms are adorable and I love it very much.
Okay that was a long list. Obviously don't overuse any of them, cause then they lose their power, but I do believe that this is a nice little connection. Feel free to add onto it, but I can't come up with anymore just in this moment.
I do hope this inspired you in any way to write any of these in to your wip, because I find them adorable.