http://www.gamespot.com/ps2/action/gta4/preview_6111263.html
Depending on how you choose to develop CJ's character as you play through the game, his demeanor and commentary will change. For example, there's fat CJ, pissed-off CJ, wisecracking CJ, and sweet-talking CJ, to name just a few. These different sides of his personality will play out over the course game as the game adjusts your conversation style to match the situation. Unlike Tommy Vercetti, who would only respond when he felt like it, the frequency and nature of CJ's dialogue is under your control. Any time he's spoken to (which is often), CJ has the choice of responding by tapping left on the d-pad for a negative response and right on the d-pad for a positive response.
This feature wouldn't be much good if CJ didn't have people to talk to, but of course he will--a whole state full of them. You'll find over 300 different kinds of pedestrians in the game, with 60,000 different comments. The colorful residents of San Andreas like to talk to CJ about all sorts of things, like their daily lives, and their opinions about nearly anything. And of course, everyone is a critic. You'll find that every pedestrian walking the streets and country roads of the state has an opinion about CJ, and they'll frequently comment on most aspects of his appearance--everything from his choice of clothing, haircut, and tattoos to his physical fitness. Their comments range from fawning to blistering and everything in between, and they'll even turn their heads to make sure that CJ hears their snide remark.
However, all the jibber jabber in the game isn't just about CJ. San Andreas is a whole state, after all, and people have lives. The locals are not limited to just taunting CJ or throwing out random observations. Citizens and illegal aliens of the state can and will, for the first time, talk to each other about their lives. These conversations can happen anywhere and any time. Observant players can pick up interesting details about life in other parts of San Andreas, like Hashbury or Angel Pine, from the conversations that happen on street corners around the state. If you listen carefully, you'll find that there's all sorts of information that will come your way.