When I write I get a lot of sensory information in my head, especially for characters that have strong personalities. I see their facial expressions and body language; I hear slight changes in inflection when they speak. My supernatural characters tend to be very heavily scent-influenced as well, but smells are actually easier for me because I can use a few punchy words to get the effect that I want, and who's going to tell me that fear doesn't smell like tin?
Normally you'd think this all this visualization is good, but at times it can be a total PITA. One of my favorite characters, Gabriel (who has a cameo in A Wild Hunt), has a very strong personality but communicates very often by slight changes in facial expression. (Or, more precisely, he's not trying to communicate and the slight changes are all the other characters have to go on.) This frustrates the hell out of me. One of his favorite expressions looks like this: one eyebrow raised, head cocked slightly to one side, a smile on his lips that quirks up higher on one corner than the other, with the smile being mild or hard or thin or brittle or predatory (etc) depending on his mood.
It takes far longer to write that than it would to see it, and it's boring to read as well. And there are only so many times you want to write variations on "he smiled," "he smirked," and "his mouth quirked up at one corner."
Then there are expressions I can hardly put into words. You know the expression people get when they're weighing one thing against another, and their face screws up, their eyes squint, their shoulders hunch and they either make a weighing gesture with both hands or hold one out flat and tip it from side to side, usually while saying "eh"? I hate trying to write that expression. I usually end up giving the character bits of dialogue that they wouldn't normally say, because (again) there are only so many times you can write "he considered" or "he shrugged," and they just don't convey quite the same information as the visual. Sometimes it's just not worth the effort of trying to get that across.
What are some of your favorite expressions? Which ones do you have trouble writing? Which do you never want to see in print again? (I'm betting "she bit her lip" is on that list…)