LOL Jen! I do that all the time! And in testament to that fact, I'll share a little embarrassing moment with you. I was alone, so it wasn't *that* embarrassing, but still...
I'd been writing Lois for days and I was getting out of my car and stumbled over something. I said aloud, "Good one, Lois, good o.. ohmygod. I'm retarded."
I do find that it helps to write when you're actually in the character's mind... or as close as you can get.
Sometimes I have trouble writing Lois's reactions when the conversation is in Clark's POV so I have to almost re-write it in my head from Lois's POV so that I get her reactions to ring more true.
Sara (who's off to do just that... <g>)