Well, how a bible study is ran depends on the ones hosting it. Some it may be ran more like a small church service with one individual teaching. I've seen some that bring in music for a short praise and worship, but not most.
Some of them are more discussions. Perhaps one person in charge just to make sure things stay on track, but not really the sole speaker.
Some the host may have a primary speaker but they may call on people, or open chances for comments or questions or answers, like a school setting.
It really varies. Often times smaller groups tend to be more of a lead discussion while bigger groups tend to have a teacher.