Do you have any older brothers or sisters? Job interviews are one thing, but when it comes to talking to people who firefight and rescue, what you need to understand is that you are potentially talking to your future big brothers and sisters. Focus on that and be comfortable with it. You have no reason to be nervous about being yourself.
Be nothing but brutally honest about whom you truly are. Don’t be afraid to say you are weak or human in areas where you are weak or human. All real firefighters are. Your fire, rescue, medical and technical training will come, but what you want to do for now is show your future comrades that you are an honest, dependable, team-player who is stable and level-headed enough not to pretend to be anything that you know you are not. Among firefighters, being strong willed and minded enough to say “I can do this, but I cannot do that” is a critical, critical, key trait that many wannabe posers do not fathom.
So, relax, be yourself and approach it as if you are going to talk to older brothers or sisters. You’re not going to out-smart them or out-think them; and you do not need to! Bravado will not impress them. Neither will saying you want to be a hero or that you live only to be a firefighter. You want to be prepared to tell them how and why you know you can be a good addition to their family, team and organization.
Prepare yourself to answer how you have handled critical incidents in your life’s experiences and how you have and plan to handle new kinds of stress, particularly, long-term stress. They will want to sense your ability to deal with this. It is far too expensive and punishing for a fire department to invest the time and resources in a new member who loses his edge after the first time he has to run down the road to scoop up what’s left of a child who was not properly strapped in a vehicle and thrown several hundred feet through a vehicle’s windshield, or the feeling you get when the better part of what had been the flesh and skin on a burn victim’s hand or lower arm sloughs off in your glove as you try to help them move to safety, or the reality of all the smells and fluids associated with successfully and unsuccessfully doing CPR on a full blown code blue call… Hopefully, you know you cannot “tough” your way through these sorts of situations; be ready to openly discuss the different ways you would handle the post-trauma stress from events like these. Like an older sibling who threatens to kick your butt, they will know the difference between nervous pup energy, piss and vinegar, and any hint of panic-freeze or run-hide type anxiety you show them. You want to show them that you are fearless of being fearful and human, not that you are a fool.
Prepare yourself to tell them what you are going to do, if they do not offer you a position this time around. Have a couple nearby volunteer departments scoped out that you can talk about joining and helping and learning from or local college fire service or medical credits that you plan to pursue while you maintain your current job and await the next opportunity to prove yourself in their eyes. Give them the chance to see that you pay attention to detail and that you do the mundane too – there is no shortage of not-sexy fun to being a firefighter. In other words, be prepared to show them how you think your plans through and have alternative options in mind. Having, verbatim, a kick-ass chili recipe and a couple fall-out-of-your-chair laughing jokes ready to quote them, won’t hurt either! 
Be yourself and you will do them and yourself best. Good luck!