In written test your analytical, quantitative / qualitative skills and domain knowledge have already been tested so now you will be tested on your other qualities like leadership, interpersonal skills, body language, managing capacity to control situations, managing people etc. will be tested.
Group discussion is not a debate but a discussion on assigned topic. You get a golden opportunity to market yourself and exhibit your skills.
GD is in fact very informal and group members get plenty of chances to recover and take up Issues of topic and conclude in an acceptable manner.
Simple approach is to be very positive for GD so think for a while before you lead the discussion because opening sentences mater the most and you can set the agenda for GD. Conclusion being most impressionable part of GD must be taken up with maturity and responsibility.