What do you find the most complicated thing to describe when writing?
As Mare said, I do think subtle movements, facial expressions, and body language can be hard to describe. In real life, those are nonverbal cues that we interpret without needing words, so trying to put them into words can be difficult.
I also find it challenging to describe performances - any scene that involves singing, dancing, or playing instruments. It's hard to find the words to describe something that is really meant to be heard or visualized. That is why if I'm writing about a real performance or song, I will often include a link to YouTube. It's like, "Okay, here is my interpretation of this performance... now go and watch the real thing so you get it." That happened a lot in Curtain Call.
When you decide to use song lyrics, quotes or anything else as a header for a chapter, do you tend to pick that first and then write the chapter around it or after the chapter is written do you go in search of the best lyrics, quotes etc... that would best fit what you just wrote?
I don't do this very often. Actually, I think the last and only story I've used song lyrics as a header for every chapter was Years of Grace back in 2001. For that one, I came up with what the chapter was going to be about first, then picked song lyrics that would fit it.
For Song for the Undead, Rose and I start each chapter with an excerpt of a journal entry written by whichever character the chapter centers on, and it's funny because the way we go about writing those is completely opposite. Rose usually writes the journal entry first, knowing what the chapter following it is going to be about, and then writes the chapter to fit the tone of the journal entry. I almost always write my journal entries last, after finishing the chapter. I usually don't have an idea for the journal entry before I write the chapter, but when the chapter is done, I can go back and mine it for ideas and find some theme I can write about in the journal entry.