Yeah, I've written a few stories for that pairing. What really helped me was stopping to think about what drew them together in the original show, that push-pull thing they had going. It wasn't just chemistry; it was two stubborn people who saw right through each other's defenses. So in a fanfic, you can't lose that friction just because they're together.
I focus on small moments. Maybe Arnav cooks something from his past for her, and she notices the gesture means more than he lets on. Or Khushi, trying to 'fix' something at work for him, accidentally uncovers a vulnerability he keeps hidden. The emotional depth comes from them having to be vulnerable when they don't want to, not from big dramatic monologues. The original show gave them such specific defensive mechanisms—his anger, her naivete—and having them carefully dismantle those for each other, scene by scene, feels authentic.
Honestly, reading a few angsty one-shots on Archive of Our Own gave me a better sense of pacing than any guide. You see how other writers stretch a single misunderstanding into a whole character study.