Basically, you need a controversial topic to start. Once you have established a controversial topic, divide your students into two groups; those who agree with the statement and those who disagree. They now prepare their arguments. Once you have done this, arrange your chairs so that there are two hot seats facing each other and then place chairs behind each of the two hot seats (enough for all of your students).
The idea is that two students start the topic of conversation, trying to defend their group's point of view. Once started, you then tap any two students on their shoulders during the conversation (Always one who is in a hot seat and one who isn't) Once they have been tapped on the shoulder they MUST stop the conversation and two new students must resume it exactly where the other two left it, even if this is in mid sentence (they change places with the person in the hot seat). They must make it coherent and follow the previous opinions and statements! They must continue the sentence of the previous speaker exactly where the previous student in the hot seat left it!
I like this activity especially because it involves all the students and they can't afford to sleep on the back seats because they know they will wreck the lesson if they do!
One other point: pre-teach some useful vocabulary they can use prior to doing it. For example, the vocabulary associated with the topic or which people use in debates e.g. I disagree, I think you are right, In my opinion, to be honest etc.