1. Talking too much
Starting a discussion creates a good impression for sure but if you talk endlessly then you are bound to speak out of context and lose points for it.
What to do: Speak first, speak only on topic given
2. Keeping silent
If talking too much can kill your points, saying nothing ruins everything. Even if the topic is out of context or out of your understanding, try to make some good, valid points there.
What to do: Assert an opinion right from beginning, talk without hesitation.
3. Not letting other speak
Group discussion doesn't mean that you will not let others speak. Be interactive. Appreciate valid points, respect antagonistic views, keep your points further and encourage a debate around it.
What to do: Use phrases like, I appreciate, I second, I also have the same view point.
4. Being loud
Speaking doesn't mean you have to shout your lungs out. Business partners often respect those who put across their views firmly but politely. You will be judged on those parameters.
What to do: Practice your tone, pitch and pronunciation before the interview.
One is expected to be familiar with his domain and current affairs. Follow news dedicatedly so that you have informed points to present if need be. Try to talk to your seniors about their experiences.
What to do: Be prepared for common topics in advance