Most TAs try to be very casual and approachable from the start. You want the students to respect you and be considerate, but you also want them to feel comfortable with you.
It is important to get a feel for your audience, to help you judge the appropriate level of your explanations, and to help recognize clues that they're not catching on. The students often have a hard time asking questions not because they understand it all, but rather because they don't understand enough to know what to ask. Be patient if you expect a response; they're hard to get at first.
It is usually best to aim discussions at the median students, or a bit above. There's no harm in filling in details, and it's a good idea to show a new example that illustrates a concept they supposedly know. As a check to see how much the majority can handle, try throwing out a new problem to solve. If you get no response, break it into simpler and simpler parts until someone will be brave enough to suggest a solution. You may have to suppress some eager students in order to avoid letting them push ahead too fast for the others. As they work through the remaining parts and put the parts together check for boredom or interest (eye contact helps). If they're bored, they may know their stuff, so move on. If they're interested, you successfully exposed the extent of their ignorance, so they may even have learned something. It really is valuable for them to see how you go about breaking a problem down, so go ahead and narrate at first, leaving more and more up to them. Not only will you be able to see what they can do, but they'll be able to spot what they can and cannot do.
It is perfectly fair to defer questions aimed at too high a level for the rest of the class until after the session is over. Likewise, if one or two students are clearly being left behind the rest, see if they can stay after.
It may happen that you plan to cover several topics and that you end up spending a whole session on just one. For this reason, you should weigh which concepts you will cover. If you are forced to choose between presenting two things try choosing the one you feel is more important. If you feel all of them are important but there is not enough time to cover them, this is something the professor should know. It usually means that the class is going too fast and that the students are getting lost.
Discipline is not often a problem, but if it is, one approach might be to remind the student that other people are here to learn and that he or she is expected to behave as an adult, etc., etc. This brand of intimidation doesn't force you into an overt assertion of authority, which of course would acknowledge that such authority is threatened. If that doesn't work, then threats may be the way to go (but after class). If you aren't sure how to handle a situation, try to downplay it during the session and discuss it afterwards with the instructor.
Finally, don't let us tell you what to do. The purpose of your classroom time is to enlighten, so remember that everyone has their own way of demystifying our field for the novice. The use of many and varied examples is invaluable. It is better to use examples not appearing in lecture (unless specifically requested) to avert boredom. Feel free to experiment but try to be sensitive to what works and what doesn't. Also, if ever there are questions (technical or bureaucratic) that you are unable to answer, the truth is best. Defer discussion, and come in with the answers at the beginning of the next session.