I liked what Sean said about it at The Comic's Comic:
Louis CK, still so great. Even when he's clearly doing material, because the way Louis CK does material is so conversational already.
Sometimes people ask me "What do you talk about when you do standup?" The real answer I'd like to give: the same things I talk about when I'm not doing standup. I'm not all the way there but that to me is the pinnacle — to talk about the same things onstage that I talk about when offstage, when I'm hanging out with a friend or someone who I find interesting. If those things are what I talk about with someone I like and respect, but then I go and talk about meaningless bullshit in front of an audience it shows either 1) I don't respect the audience or 2) I'm not good enough to get laughs out of the stuff I truly want to talk about. Either way = kinda shitty.
Also, I think hitting that level of conversationality (?) comes from how you write. When you manufacture jokes like they're math problems, they come out sounding like jokey jokes. When you go onstage with an idea and just talk it out and find the laughs organically, it winds up sounding like a real conversation.
Or, when a bit is based off a real conversation with someone, that has a similar effect. Sometimes I find the best way to write material is to go get drunk with a friend I think is smart/interesting and carry along a notebook for anything standup-ish that pops up.