The Voices in My Head Say Strange Things Sometimes

This semester, I am taking a class called Personal Essay. It's exactly what it sounds like - a form of creative nonfiction that essentially encompasses a mini autobiography (so instead of telling you about my entire life, I get to tell you about this one time at band camp... (no, I was never at band camp, I just couldn't resist)).

One of the things my professor talks about a lot is dialogue, and how to make it sound natural. We did a free-write style activity where we had to chronicle a recent conversation we had had, making all the people involved sound distinctive and, well, like themselves.

IT WAS SO HARD.

I wrote down the conversation I had with my made-of-awesome roommate when I was freaking out over an Italian presentation I had to do. And I could get the me-freaking-out part just fine. But the her-being-reassuring part? Needless to say, I did not volunteer to read it aloud to the class.

After a few brave people did volunteer, my professor asked us what we thought about the exercise. I told him I thought it was really difficult, because I am used to fiction, where you literally know every facet of your character. You know what they are going to say, how they say it, how they sound when they say it, and why they say it.

Even if, as my professor suggested, you write down or tape conversations with others to get their mode of speech right, there won't ever be a time in real life when you know all of that. I don't think I could even get my best friend down perfectly.

Goodness, but real people are so difficult. :P I find it easier to make realistic sounding dialogue out of nothing than to take a real conversation and keep it from sounding fake. How weird is that?

So what I would like to know is this: how do you approach dialogue? Do your characters ever say things that surprise you? What do you do when they start sounding, well, not like themselves? Do you plan out every single little thing they are going to say? Dialogue is important; it shows an awful lot about a character (or, in the case of my class, a real person). So how do you guys go about creating realistic sounding dialogue out of nothing?