Dialogue comes easily once I've got the character in my head. Once I know what they want, and how they feel about it, and the people around them. That's the tricky bit for me...

It depends on the genre I'm writing. Comedy is an easy genre for me, especially farce, so the kind of patter comedy dialogue that, say, Stephen Moffat writes in Coupling?--I can do that. But if I take the same characters with the same personalities, goals, and flaws, and put them in a drama...um, I'm using up a lot of energy to stop them from being funny. I know: give in and let it be a farce, right? I generally do. But sometimes the story really wants to be a different genre and then I've got my work cut out for me. I have a friend who is a genius at writing dialogue. Sharp, distinctive character voices, very real, very funny. So I compare my work to hers to keep myself honest. If anyone ever mistook my writing for hers, I'd think: wow, I'm doing something right. I like having people around who raise the bar. I have another friend who can tell stories about his own (bizarre) life and perfectly reproduce other people's accents and speaking styles. I can't do that and I envy him. But following his example, I try to "tune up up my ears", and listen to the people around me (not TV or movie people! real people!) to grok and retain their speaking styles. It's the 80/20 rule. I'd say that first 80% of dialogue being adequate to even good comes easily enough to me, but closing the gap of that last 20% to be really excellent is where the work and struggle happens. I were I were a natural 100%er and I'm not, but I'm happy to hang in there, study, rewrite, and keep trying to close the gap on any individual project and get better in general.