Students can be warmed up with simple questions from you as teacher leading finally to the subject that you want to discuss. Finally the groups can be brought together into a more general class discussion. Better have a gap-filler or alternative activity up your sleeve in case the discussion dries up. In roleplay activities, students in pairs or groups are given specific identities in a particular situation and asked to act accordingly. They are usually given quite detailed written instructions.
Many course books contain photocopiable roleplay instructions for a variety of situations.
And with a little imagination you can easily prepare your own. Try to given participants conflicting objectives. The students form a panel of judges who have to decide the winner of a competition. Give each group five or six short poems or photographs to judge. Then let all the groups come to a unanimous decision.