Getting things right on an emotional level.
I'm currently working on my second Doc Vandal adventure, and have somehow found myself writing an interracial lesbian romance as the B-story.
As a half-outliner, half-pantser, I have to start by saying I didn't plan it. My female lead character encountered another woman in a pretty dangerous situation, and as I started writing the dialogue it felt right for the character but not entirely right for the situation (based on the new character being a one-scene-and-done as originally planned). Eventually, I figured out it was romantic attraction, which created an entirely new character arc for the existing character.
So now, the hard part is making sure I'm emotionally true to these characters and their feelings. As much as possible I'm trying to focus on the two characters as individuals, rather than to try to put them under any sort of label. It also doesn't help that the POV character of the two isn't really thinking in those terms yet, mainly because sexuality has never been a big part of her life. Several other characters know exactly what's going on, but she's basically clueless. (I'm coming up on the scene where she figures things out, but it may be a bit before I can put down the paying work to get there.)