Pathos is the key to a betrayal story, and in order to have pathos, you have to start with characters who have a believable relationship. They don't have to be lovers - friends or family members can also work - but they have to have affection for one another. And trust. You want your readers to feel the same sting as the betrayed character, when the MC'ed character betrays that trust. For me, it's even hotter when the MC'ed character is so far gone that s/he doesn't even remember there was a relationship, but that's just my personal kink. It's not necessary for the MC-ee to be completely mindwiped; s/he just has to be far enough gone to place obedience ahead of relationship.
So, how do you do that? Well, whether you're writing a story or doing something with a visual element, the same basic rules apply.
Start with body language. The key here is subtlety. If your characters are lovers, don't show their lust for one another; show their love. Have them caress one another gently - on the cheek, perhaps. Show them holding hands and smiling softly at one another. If you really want to put them in bed together, be at least semi-discreet about it. Show them cuddling, legs wrapped loosely around each other, that sort of thing. The idea is to make them more than just fuck buddies. Being betrayed by a fuck buddy doesn't hurt nearly as much as being betrayed by someone you love.
There are other ways to show relationship with body language: ways that are just as appropriate for non-sexual relationships as sexual ones. Have your characters stand or sit close together. Give them eye contact. Position them so that one person's hip leans toward the other's. Place a hand on a shoulder. Let them smile at each other, or even better, give them a legitimate excuse to smirk. There's just something about smirking that implies closeness.
Now let's move on to dialogue. What is it that the characters share? Suppose they're longtime friends and coworkers. Don't tell your readers they're longtime friends and coworkers; show them. And again, make it subtle. If these were real people, they wouldn't say things like, "You know, we've been with this company for twenty years now and you're the best friend I have here." (Well, they might, but it would sound really clunky and artificial in a story). Let them talk about something more personal while they're working, or let them talk about work while they're doing something more personal.
Now suppose your characters are lovers. You can let them call each other pet names like "honey" and "sweetheart," but that alone won't sell their relationship. What do real-life lovers talk about? Ordinary, everyday things. Their relationship comes through in the way they do things together. For instance, maybe they're talking about buying a new house, or planning a party, or visiting one or the other's parents. Again, strive for subtlety and naturalism.
This technique is easier to demonstrate than explain, so let me give you an example from one of my favorite Tabico stories, Allegiance. This bit combines body language and dialogue, so I want you to notice both as you read:
They looked at each other, drinking with their eyes.Notice how much Tabico has told you about the characters' backstory without spelling any of it out. They're soldiers. One's retired, and the other one's still active. They're lovers who haven't been together in 20 years, they're aching for each other, and yet - this is critically important - their relationship isn't just about sex. If it was, they wouldn't be talking over drinks; Ellibree wouldn't be probing so gently about Tetha's hurt; and Tetha wouldn't be willing to spill classified information to her. They'd just be screwing.
Finally, her guest sighed, and looked at her glass. She swirled it around, clinking the ice, and looked back up.
“Tetha. I’m... I’m so happy to see you.”
“I’m glad to see you, Ellibree,” Tetha replied. “It’s been so long.”
“Twenty years,” Ellibree replied. Spoken, the number hung in the air, heavy and accusing. Ellibree shook her head.
“A very... long time indeed,” she began again. The words felt false, brittle. Flimsy boards over a tempestuous cavern, unable to hold for long. “How have... you’re out of the Service?”
Tetha nodded. “A month ago. Retired as a Colonel.”
Ellibree smiled. “Higher than me.”
Tetha’s answering smile didn’t touch her eyes. “Rank doesn’t mean so much, what I’ve been doing.”
Ellibree nodded, suddenly sober. “Can you... are you allowed to talk about it?”
Ice glittered in her glass as Tetha turned it, seeking something inside. “No. But I’m going to.”
But you wouldn't have nearly as hot a betrayal story.
I hope you all enjoyed the tips here, and that you can use them in your own work. Thanks to Uzo Bono for asking the question. And if anyone else out there wants to know more about the writing process, I'd love to hear from you - and maybe address the topic in my blog.