Before you can successfully implement a discussion session, you will need to become aware of the implicit set of attitudes and messages you bring into the classroom with you. Equally important are the attitudes and expectations that your students bring with them.
You - Your reactions, your responses to students, the attitudes you project in your actions all suggest to your students the sort of interaction they can expect. The way in which you field students' comments will give the most important clue. No one wants to feel that their remarks will be put down or put off. Students are also sensitive to what they think you REALLY want (e.g. does he want a discussion or a chance to give a mini-lecture on his favourite topic? Does she say she wants disagreement and then gets defensive when someone challenges her?). Your students will try to read you so that they can respond appropriately. Be sensitive to the clues you give them and do your best to create a 'safe' place for open and frank questioning and discussion to take place.
Your students - It is well worth the time and effort it takes at the beginning of a class, with a new group of students, to find out what they are expecting from you and the class. You could simply ask them and some confident students may respond helpfully. Better still, you could ask the students to write down some brief notes about how they see your role and theirs in the class and what they see as the purpose(s) of the class. This would also provide an opportunity for students to explain privately any special arrangements they may need in order to participate fully. If your students are first years you may even wish to facilitate a discussion about how you will work together. Some GTAs find it useful to draw up class ground-rules.
"I tried using ground-rules last session and was pleased by the students' reaction. I gave them a list of 5 points, things like 'we will start on time', and I asked them to edit the list in pairs. They added some really interesting things, "no one should dominate the discussion" and "everyone should do the core readings each week". I think it helped us to get off on the right foot."