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If at all possible, do your studying ahead of time. Read any materials that have been distributed. Do any reading that was assigned. If you just have a general idea of the topic, spend some time researching the topic. In fact, it's a good idea to bring some notes with you, so you have something to refer back to in the discussion, particularly if you have evidence you want to discuss. Sometimes, people may not express themselves as well as they want, or you simply may not hear all of what they say. It's fine to ask for clarification, particularly if you want to provide a counterpoint. One of the benefits of group discussion is it gives you a chance to interact with people you might not normally have a conversation with. That means you have a chance to learn something new and expand your horizons. However, if you focus on sticking to what you believe, you may shut out any possibility of learning something new.  When someone presents an idea that takes you aback, think a moment before responding. Try to put yourself in the other person's shoes to see where they're coming from. If you don't understand something, ask a follow up question. The other person will likely appreciate that you are interested in what they have to say. A group discussion should involve everyone in the group, including yourself. When you have an opinion or a fact that's relevant to what's being said, jump in and voice what you have to say.  If you're a little shy, try getting the leader's attention so you have a chance to speak. You can also ask questions to encourage discussion. When you're discussing an issue, particularly in a classroom or work setting, you'll need to have evidence or research to back up what you say. Of course, that evidence could be your experience, but you do need to be able to explain why you are presenting a certain side.
Come prepared. Get clarification about things you don't understand. Be willing to learn from others' experiences. Speak up when you have an opinion. Back up what you have to say.