Descriptions. I spend so much time looking for the write word or synonyms of a word I've already used or just pulling hair out because the scene feels like it's far too short and needs more detail and care. Action scenes and NSFW scenes are what I suffer with the most, though as much as I love writing dialog since it comes quite naturally to me, any dialog scene with more than three characters is an absolute mess to plan. Too many voices.

Characterization is a case by case basis for me. Some characters are harder to write than others. (For me, Yuji and Satoru are pretty easy, but Megumi and Suguru don't.) As for research, it really depends on how easily I let myself get distracted by what I'm reading about. XD