As far as romance goes, I can actually talk from experience for this. I’ve been in similar relationships, and a lot of the ways we would show affection would be in just very subtle touches. Think about the way you interact with your friends and the way you see other people interact with their friends. From what I’ve seen, guys tend to be less touch-y than ladies (my lady friends and I will lay all over each other and none of us mind, but guy friends are definitely less into that). Once you’ve sort of identified the way you and others interact with close friends, think about what separates those interactions from romantically-influenced interactions. There’s not a lot, right? Once you take out anything sexual, there’s not a lot of difference. Like you need it to be, a lot of it is subtle.
Casual touches are big. It doesn’t have to be hand holding or hugging, but just something as simple as putting your hand on a person’s arm when you’re talking to them or gently shoving them when they make a joke. We only touch people we like or when we’re trying to gain control of a situation, so in the right context, this is a nice way to show off some affection without it being too overt. Sitting next to each other and just keeping in close proximity in general also shows some companionship.
Another way to show affection is just with the way they talk to each other and the things they say. Depending on what kind of person you are, you’re going to act differently with people you’re close to. Personally, “piece of shit” and “garbage” are terms of joking endearment for close friends, but I would never say that to my SO. That’s simply not a joke I’d be willing to use with them, because the romantic aspect changes some of those small ways of interacting. A lot of people are gentler with their SO than with others, so even if you have other characters making off-hand comments like “You gave me shit for that last week, but now LoveInterest does it, and you’re not gonna say anything?” shows that there’s something a tad bit different about their relationship.