The interview was really painless -- many of the people at the company had already known me for a long time and I was asked to submit a resume and application when they found out I was ready to change employers and career directions. I had a 1:1 interview but it really consisted more of the "interviewer" talking to me about the company, how it was structured, how positions were graded and then asking me some questions about salary and work expectations.
I already had an excellent view of the people, office and work culture as I'd worked or associated with many people from the company for years. One of the main reasons I interviewed with MITRE was that I'd already seen that people there seemed to be happy, beyond competent and productive with a good balance between life and work.
For other candidates, try talking to a current or former MITRE employee to get a feel for the corporate structure and environment. Make sure you address differences in environment and culture in different locations -- Boston, DC, Huntsville, LA, etc. all operate somewhat differently.