Ensure you have read up about the company and it's values. Be nice but assertive without being over bearing or controlling. Make sure you contribute to group conversations and be articulate. If you cannot get your message across in the group interview, how can you get your message across to any potential customers.
The most stressful part as a casual tm at me is that there's no work security coz anytime they can get rid of you legally when they don't need you anymore
I can only speak for the interview process to gain entry into the call centre. It all goes through a recruitment agency and the interviews start as group interviews then go to one on ones in the same day. It's not daunting and the managers try to make it as fun and relaxed as possible.