Robert,
I've designed and run Case Study Interview sessions in the past. They are usually unique and have the objective of sorting out the 'I can talk about it but cannot do' candidates from the 'I can walk the talk candidates'.
If you fit into the latter, relax, you know your stuff. The case study will be like a real exercise and should be no problem.
If you fit into the former, a good prayer may help.
Regards
Mike