I envy writers who have dialog flow from their fingertips onto the screen. I've been struggling today and yesterday with two scenes, both of which are dialog heavy. I've written, tightened, added, tightened again, changed the pace, change the flow, changed the speakers, changed the voice. Every time, it's an improvement, but it sure would be nice to have it come out perfectly the first time.
I guess I could say the same about all writing, but dialog has always been a struggle for me. There's so much to wrap up in the words. I try to capture the voice of the person, their mood, their speech style. Description can take a little more time, creep up on the picture or throw it in your face, changed by the tone of the scene, but dialog always needs to reflect the character.
I never paid much attention to my dialog in writing until I wrote a screenplay. The only real thing in a screenplay is the dialog. And mine was atrocious, bland, long-winded, and interchangeable between characters.
I've given it a lot of work in the intervening years and studied some of the masters (like Evanovich and MacAlister). I've gotten better. I still think I've got a ways to go before it flows easily for me.