Its hard not to agree with, some of your sentiments regarding the interview i.e "dont get too stressed", "go there with confidence" and "you dont need them, they need you".
Where i would disagree is in your comment that "anyone can do the job". I don't believe that just ANYONE can. While the majority of cabin crew would agree that day to day tasks do not require a huge amount of 'brain power', it takes a special kind of someone to look at customers as individuals rather than bums on seats. We also know that in the event of experiencing a emergency the ability to work as a team, manage situations as they unfold and put others safety before your own is paramount. Thats why airlines dont just pick ANYONE to do the job.
Go to the interview, yes have fun, yes use it as a great way to meet new people and make new friends. My real advice would be dont forget the real reason that you are there, to work for a company that breathes customer service, individuality and professionalism. If you remember that then you wont go too far wrong.
Good luck and safe flying