Roles are a clear way to set up and organize the type of talk you wish to take place. Give clearly defined responsibilities within the group talk. By giving roles you have better control over what happens when the pupils get into groups.
Once the children have labelled themselves A, B, C you can allocate roles such as ‘A you will start the talk. B you will look for evidence or continue to build on the ideas from A. C you will summarise the ideas, etc.’