I am talkative with people I'm comfortable with, but less so with others. A lot of people have told me than when they first met me they thought I was a quiet person and then they discovered after they got to know me better that I never shut up! Or with coworkers after we started chatting online they said I seemed like a completely different person (I work with computers and at both this and my previous workplace, a lot of communication was done through chatting for different reasons). When I get going, I've had several friends/family members start making chicken noises or doing that thing with the talking hand when I'm babbling a lot. I "babble" as they put it a lot more when I'm excited or interested in something. Also if there are a lot of people around, I don't talk much then.
However, I'm much more articulate in writing such as on this forum than I am speaking. Speaking is the hardest form of communication for me. If I'm just talking with a friend it's usually okay but when I have to discuss something that is important, etc it gets much harder. Sometimes I just don't make any sense at all, even to myself. Or I find myself talking in circles, sort of refining what I have to say over and over because it seems like the other person isn't getting it (they usually aren't, they just want me to shut up already). Some time ago I started writing things out if it's something important or emotional that I have to convey. It's much easier to order my thoughts in writing and make sure I've covered everything, plus, if I start to repeat myself in writing, I can edit it down and condense it.