Fair question. Most of the time, I tend toward conversation. Toward, but not all the way to. It's too easy to slide into a character voice from either my normal voice, or from another character. So I always do at least 2 takes of every line. If it's a tough line (emotion-laden, scene climax, or shouting-- which brings up technical issues) I might do 10 or 15 takes.
A couple things cause me to occasionally go the other way, and do all the lines in a scene for one character together. If the voice is really taxing (for example, extremely gruff like my Injun Joe voice in Tom Sawyer), I will record it all at once and give myself a recovery break. Otherwise it affects my voice for the rest of the scene. The other is if I get to the end of the chapter (or some later period in life), realize that I didn't like the voice for that character, and decide to go back and re-record all the lines and patch them in. That exact thing did happen in The Eli Event. After discussion with the author, we realized that one of my character choices wasn't going to work for the way the character acts throughout the book, so I went back late one night (it was not dark and stormy, just late) and created a different voice and read just those lines over again.
Speaking of the Eli Event, here's a chance to check out what happens when you have a scene with 8 characters in 5 minutes. Enjoy. And upon your enjoyment, go to audible.com and pick up a copy of the whole thing.