Hi - I work at English for Asia, so I hope you don’t mind me adding a comment here:
As M Khan points out, the pre-interview task and interview are designed to help us decide whether or not a person can succeed on the course. After all, it’s an expensive, demanding and time-consuming course, with rigorous standards set by Trinity College London.
During the interview, many of the questions will revolve around your written answers to your pre-interview task. By asking you to reflect on your answers, we try to determine your level of Language Awareness (how much you understand about the language). Those who have learned English as a second language have the advantage of having studied English grammar and can sometimes possess a better awareness of the English language than a native speaker. Good preparation for these questions would be to read over your answers and check them against grammar reference books.
You will also be given a speaking and a writing task, to ascertain your Language Proficiency (how well you speak and write).
You may also be asked simple questions about classroom/teaching scenarios.
In addition to the above, we will try to determine how well you are likely to cope with the pressures of the course and how well you could work in a team (both are essential skills to ensure you cope on the course).
Good luck!
Adam