My advice would be to be honest, enthusiastic and show a willingness to learn. Make sure you listen to what they say and ask. Before the interview think about how you might answer some difficult questions like "why should we give you this job?", "give us an example where you have show initiative", "what are your weaknesses", "how would the people who know you best describe you". And smile too.
If it were a real interview, I would suggest you do some reasearch about the organisation before hand and try to speak to someone who works there, but that's probably a bit too much for a mock interview. If you were REALLY keen you could read a copy of "Fire Prevention Fire Engineers Journal" this would give you some insight regarding current issues, but again, for a mock, that's probably a bit over the top.
Good luck.
