The interview process is extremely drawn out. After first filling out the questionnaire, which you'll likely have to lie on just to proceed to the next step since pretty much everyone else does. As long as you're not telling tall-tales you should be able to discuss everything in the interview.
(As a side note, the NPS, and every other government organization, is required to advertise ALL jobs to the public. Unfortunately, when Parks have "pet" employees they often tailor job questionnaires to fit them. If you run into a posting that asks if you "Have at least 2 years experience studying the yellow-legged snail-sucker of Timbuktu, Rhode Island, then don't bother applying, they've already selected their man.)
IF you qualify, and often this seems to be completely arbitrary, AND the Park in question hasn't already picked a current pet employee to fill the job, AND there aren't too many veterans applying, AND if the stars are aligned correctly, you might just get "referred".
Next you'll wait for either an email or phone call requesting an interview. Don't get excited, more often than not nothing will happen and you'll never hear from anyone.
If you DO get an interview it is nearly always over the phone and most likely will be during somewhat odd hours due to the many time zones in this country.
You may talk to just one interviewer, or several. Be prepared with knowledge of the park and some questions for the interviewers. The attitude of the interviewer ranges just as much in the NPS as in the real world, you might get friendly people, jerks, or people who would rather be doing anything else.
Just be yourself in the interview, I've not run into any instances where the interviewer tries to trick you or otherwise mess with you. They are quite interested in finding someone who has literally been doing *this exact job* for years, so anything and everything you have experience in is fair game. Most interviews come off friendly, informative, and productive. It can be very difficult not to get excited that you might be picked.
Next, be prepared to wait a while for a call back. If you're not picked, you will NOT be called or otherwise informed in any way. Occasionally you may receive an email stating that someone else was selected, but only certain agencies seem to do this.
If you ARE selected, you may not know for weeks or even months. My last job waited for 3 months after my interview to offer me the job.
After that will be a whirlwind of background checks, physicals, and forms upon forms to fill out. before long you'll start your first day. A bit of warning, though: have some cash saved up, you WILL NOT get paid for at least a month after starting, assuming the Park's Admin and HR departments are on the ball, this is, supposedly, due to the bi-weekly payment system and the two week lag in pay (being today for what you did 3 and 4 weeks ago.)