I applied through a recruiter. The process took 4 weeks. I interviewed at US Marine Corps in Sep 2011
Interview
You must be the most highly qualified individual you possibly can be. Healthy smart active. Have a high level of emotional intelligence and/or an uncanny ability to focus on a common goal regardless of circumstance.
I applied in-person. I interviewed at US Marine Corps (Washington, DC)
Interview
It was a very easy but a lot of paperwork. Interview consisted of medical check, mental health check, and a drug test. There were multiple tests that had to be taken on a computer and a very long background check.
This was a tough interview process and the screening was rigorous. It will take a long time. I recommend getting started early. This is a long term commitment. You need to be ready for it.
I have to do this for some reason to get access to the GD's website. Interview process, walked into the recruiting office and said "sign me up". Recruiter was a friendly guy, said "Ok!".