Having done transcription for a REALLY LONG TIME NOW, I can say with some authority that just about everyone says "um" "er" "like" "y'know" and "uh" a lot more than he or she thinks he does, even when he KNOWS what he's supposed to or going to say next, and no matter how many letters denoting supposed literacy he has after his name. This is why clerics have to practice their sermons-- so they won't drive everyone out the door with "um" and "annnnd..." Run-on sentences that go on for hundreds of words without a full stop are also more common and normal than many suppose, and tend to be present in "outdoor" or public speech more than "indoor" or domestic, whether the dialog is face-to-face or over the phone. Why this is the case I'm not sure. Once when talking on the phone with my dad I sat without saying anything but "uh huh", to see how long he would keep talking without a full stop-- twenty-five minutes! None of it was repeated, either... :|
Sentence fragments or very short sentences with NO "ums" or "ers" are usually present more in domestic dialog between two individuals who are quite familiar with each other, to wit: "Where's the damn milk?" "You left it out on the counter. Had to toss it." "Huh. Move your foot." "Sit over there!" (long silence) "Goin to the park?" "Just got back."
Trying to render dialog with 100% accuracy would make said dialog stretch on for page after page and be incredibly boring-- and hardly anyone would want to read it, so of course we don't want to do that.
When you are talking to a relative stranger and he says "uh..." that usually denotes simple hesitancy ("So, where are you from?" "Uh..." Meaning "I'm not sure I want to give you that information at this stage in our relationship")-- when in the midst of a conversation your homeboy says "uh...", however, that usually means he's trying to think of a lie: "Did you see Danny last night when you were out?" "Uh..."
I hope this was helpful!