Weak answers are very different from wrong answers. Remember that it is effectively a competition and the better candidate will be chosen when there is a limited level of employment.
When answering the question try to be specific and as concise as possible. Whilst they are looking for a specific response remember that it is how you present the answer that is just as important.
Nerves are a weird one as a decent interviewer should be able to make you feel comfortable and relaxed. They should also be able to account for nerves as its an interview and many people are naturally uncomfortable in that situation. I keep telling people. After application and assessment the hard part is over. Relax and be yourself. Nervous people aren't suited for the railway so its a difficult trait not to notice in people.
As a friend to interview you. I think the job centre also does interview practice too.