They are not there to catch you out, each careers office wants as many recruits passing through them as possible, so as long as you look presentable (wear a suit if you have one, make sure you shave etc), bring everything that they have asked you to bring, and show them you know what you are getting yourself in for, you'll be fine.
Don't worry about it.All they do is ask why you wanna join and what you're family situation is like and what you wanna aspire to do etc.It's all about you and if you love what you wanna do that much then you will know about your possible chosen regiment so they can't ask you anything you won't know!good luck fella!
I turned up in a suit and tie and the recruiter just laughed and said "Oooooh look at you!"... cnut
Whole supposedly formal side of the process has been really casual, interview was just a chat and actually a really good laugh. Was given my selection dates and rail warrants and told not to worry about a second interview, top bloke.