I first spoke with a rep at my school's career fair; nothing came of that until I was contacted (a month later) to schedule a phone interview for a position I hadn't initially applied for.
The interviewer's department took 3 weeks to get back to me. They scheduled my phone interview and I did well enough that later the week after they asked me to schedule an on-site.
Do NOT be fooled into thinking this is a formality and they really want you; they don't, they just have a lot of government money to spend on bringing in anyone who's even slightly interested in them. They also enjoy monopolizing your time (i.e. by making you take 3 days out of your schedule that's 3 days you can't interview with anyone else, thus more likely to want them).
The on-site consisted of a holding pen (they gather all the applicants for that day in to the side presentation room), while they set up the rooms and schedule. Afterward you're taken to a room to do 3 one on one interviews with average engineers on the team; you can't stumble with any of them even 2 amazing interviews and 1 okay will not fly.
They take you to lunch with 2 or more employees, one will in general be someone you interviewed with already (this is not by coincidence, they are quizzing you personally, i.e. "getting to know you"). You'll take a tour of the building, and the person you interviewed with will pretend they have a phone call to make or something like that. They will leave you with the person who didn't interview you, who'll seat you in the conference room and answer questions/ tell stories.
You will not see anyone you interviewed with again; they are discussing you animal house style in the main room down the side hall. They give you a software demo, when all the applicants are in the main conference room.
After the demo is over they will call each of you by name the first ones called are the most likely to get hired, but not necessarily (each potential hire goes in front of an executive, they don't always have enough there on that day).
If you don't get an offer before you leave, you are not getting hired. Period. I know some people in the valley and they've confirmed that this is in fact what is going on.
Do not believe their line about talking it over, they've already done it, you're not in.
A few days later when you submit your receipts for reimbursement they'll call you to tell you you're not hired or if you wait too long they'll email you the same spiel about how everybody likes you but you're not cut out for this position; this is a lie; they will not ever hire you; they have a list to make sure no one who's ever been through this process never gets picked for a potential hire again. People they like get hired; we they don't do not (regardless of qualifications).
I would advise you to not accept the on site interview (if you're persuasive enough they'll do more phone interviews, which if you do really well in they'll just give you an offer without meeting you, saving your time if you're just getting jerked around). If they won't work with you they don't really want you anyway, but don't get jerked around, don't go hook-line-and-sinker for this job, because the don't really have openings. They'll make an opening if they want you, otherwise there wasn't an opening to begin with.
Apply at your own risk (hopefully I've saved at least one person with this post)