In secondary schools this will be in your subject area and you should be given guidelines on the lesson for which you need to plan, with your interview pack.  In primary schools you will also be given guidelines on the lesson, or part of a lesson, which you have been asked to deliver.

The observers/interviewers will be interested in: the quality of your planning and preparation; your rapport with the pupils and management of any behaviour issues which may arise; your knowledge of content/teaching strategies; and your professionalism.