IIM GD

In any of the IIMs, GD dont count much if you have very good percentile and profile. Except for IIMA, one can get a final call even if he does not open his mouth provided he has an excellent percentile and profile. Now, people generally feel that since they dont have very good communication skills so they are at a disadvantage. Well, take my word this is not the case. The general do's and dont's for GD:

DO's
1. Open your mouth atleast 2-3 times.
2. Try bringing new points which can give direction to the discussion.
3. Since IIM GDs are really very peaceful, dont be aggresive and dont cut others.
4. Try being among the first 3-4 speakers, but again dont try to cut others to grab the opportunity.
5. Read newspapers and magazines atleast for 2-3 months. It helps!

Dont's
1. Dont open the gd if you are not sure of your points and the case/topic at hand.
2. Dont open your mouth every now and then!
3. Dont interrupt a person who is speaking for the first time and that too a very valid point :D

GD can only make your case stronger. It is not a critical factor in final selection. Dont commit any blunders. You may have a tough time with IIMA atleast if you have done some glaring mistakes.

Best of luck.
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Vikas