Dialogue is really big for me. One of my characters, her name is Robin, is the team's unofficial little sister and she loves to tease Nick, the team's IT specialist. One moment I highlight is when Nick gets mad at her for stealing one of his shirts to use as PJs. It's very clear from the dialogue that this isn't the first time she's done something like this.

I like to use "touch" as an indicator of character relationships. It's one of my love languages and it's really normal in my house if you laid across someone's lap, you'd get your back scratched, etc. There are a couple of moments in my series where the MC is cuddled up next to her dad and he starts playing with her hair. It's a simple gesture, but it helps establish a level of comfort that they don't share with anyone else.

"You're upset," Jamie observed, resting her pillow against Cole’s leg. Cole grumbled and focused his attention on the cartoons that played on the TV. "It wasn't anybody's fault. Robin and I were doing a lift and the guy grabbed me.”

"I know." He combed her hair with his fingers. "And I'm not upset."

Jamie rubbed the worry-born crease on his forehead with her finger. "Yes, you are Dad. I remember that you get grumpy-lines when something's bothering you." Cole fought back a smile.

I live for the subtly of relationships.

One of the most obvious subtle things: names and nicknames and general address! Really obvious in anime (that character who's constantly called by their family name or full name except with that one character who uses the given name, good stuff) but outside of that too. In The 1st Rule, Ari never addresses his younger brother in anything other than professional terms, even in life or death situations, but is surprisingly casual with Louis, his boyfriend, and if you read the side story you learn this distance is not a new thing, it's been this sorta one way relationship for year (it's plot important but also he's just way too professional). And also the painful version of this, the K anime, Fushimi is the only one who calls Yata by Misaki, indicating both their former closeness despite working for opposing organizations and getting him riled up because, a, betrayal and b, it's generally a more girly name. And equally, Yata shortens Fushimi's name Saruhiko to Saru, meaning monkey because a, he's mad and b, he knows Fushimi's father picked Saruhiko as his name because he looked like a monkey as a new born (and not in a cute pet name way, he meant it in a asshole way).

How "bantery" can they be? You know, some friends will just insult each other and then laugh. Some can't. Sometimes an insult to your skill is met with a smirk and a friendly match, sometimes it's a serious insult to your honour. Sometimes a bantery, naturally rude character will tone it up or down depending on who they're interacting with to make them more or less comfortable based on how much they know them. In Where The Sky Lies (very soon in the coming updates) Francis points out No 1's only personality trait seems to be he's a liar, and No 1 laughs and agrees, but wouldn't take it from anyone else.

Same with flirtiness (I love a shamelessly flirty character who just gets brushed off by everyone in his friend group and knows when to behave. The Joey, I guess), who comfortable are people with their harmless flirting, do they flirt back with the understanding that they're just friends or whatever, do they get uncomfortable and does the flirt tone it down in response?

Just simple phrases. One of my old works had character A just gently reminding B, their best friend, every now and again that "you're still my favourite person" even as situations around them changed. Another had their reincarnated soul mate just reminding them "don't panic" even when they barely knew each other. And in jokes, around a certain group of my friends "never ever" is met with a bunch of people laughing "except that one time". People's reactions to in jokes let you know how in they are with a group even if the reader doesn't get it and just like with nicknames above, if someone leaves the group it can then get painful. Your enemy is now sneering your own in jokes back at you, ruining them and letting the reader know they used to be friends.

I completely agree with the stuff about dialogue! The way characters address each other tells a lot about themselves but also about their relationship with others! (Like in the way my main characters used to use the last name of the other when they're mad at the other one, but it's more and more used for teasing.)

Then again, I'm more a person who prefers actions over words, which is something I should implement more in my own story. How people's behaviour shifts when talking to different characters is important, so are caring gestures, body language, etc. (I'm at a point in my story for example where the side character Jacob is not looking with hostility at Simon anymore, but with gratitude.)

I grew up watching my Dad stop my mom from doing dishing to slow dance around the kitchen. He loved to dance.

The other thing they'd do is when food was served they'd feed each other the first bite.

My character in my series Bright Morn of Issareth refers to his adopted son as "Son of my Heart."

I totally agree with the dialogue and the nicknames. I do that for quite a few relationships, friendship or romantic. Sometimes, I use inner dialogue with characters to do some self-reflection. I particularly like doing that with characters that are more stubborn and trying to work through their feelings that they've been hiding or didn't even realize that had for someone. For example, one of my characters, Penn, has this back-and-forth with another, Jack, and when she thinks he's into someone else or has 'a moment' she starts to implode a bit, but doesn't really know why. But we know why!

I might hold you to that!? I MIGHT HOLD YOU TO THAT!?!? I screamed at myself. Where the hell did that come from? And why did I mean it?

I mean, I know we seemed to be having a moment, but… what the hell was I thinking!? Jack is like my nemesis… right?

“Ugh!” I groaned as I walked up the stairs to my floor.

I don’t know what to think anymore. Trying to be me and Felicia was getting confusing. I was hoping to have fun with this secret double life NOT to land myself in the type of drama that Lifetime movies were famous for.

I like long-term mannerisms like having one character be less touchy than the other, and the other picking up on it and respectfully not trying to touch the character until they initiate touching them first :3

For me there are a few things to consider about this, and it really depends on where you see the relationship going. If they are meant to find love in other individuals, things that are considered romantic would not be attributes I would apply to their interactions. Instead, the dynamic would be something that showcases their strengths while at the same time showing why each other is important to the story. The movie, the House with the clock in the wall, demonstrates this dynamic really well for me. The characters do seem to create a family bond of some kind, something that really isn't nailed down, but it doesn't need to be as the movie comes to an end.
For individuals who are meant to find love with each other, the trick to making the relationship work are the subtle ways to keep things interesting. For example, if you're creating a scene where he/she can't stop thinking about somebody, what odd physical traits do you exhibit, or have you seen people exhibit that show tension, anxiety as they think about the next interaction.
For general friendships and work place relationships it really comes down to solid character development. If I can't connect with a character then I won't invest my mind into their actions. They need to seem real and genuine, rather then purely created to serve the story's intended goal. Don't just right an idiot or comic relief, right someone who is a bit opposite of the main viewpoint character, but serves a purpose. Don't just right a love interest, write a woman who recognizes her limits, but faces challenges in a unique, or doesn't face challenges because of her back story. These clues really give readers what they need to latch onto as they progress in the story.

Touch is a big one for sure! Also eye contact--or the lack of it--can tell a lot about the dynamic between two characters. I think eye contact is the most powerful form of nonverbal communication. Windows to the soul and all that.

Between the main couple in my romance story, direct eye contact and held gazes show that they're engaged in their conversations. It can mean that someone is important to them, or that someone is saying something important. Maybe your character is averse to eye contact with the people they feel uncomfortable with. Lack of eye contact can also indicate disrespect and defiance. Maybe someone is staring at their wine glass instead of directly speaking to a person? And lowered eyes can indicate deferrence, and so on.

I probably rely a little too much on it though, and this thread is giving me great ideas on other physical body cues.