Contacted by third-party recruiter over the phone and received short (20 minute) preliminary interview on the spot. Moved on to second interview the next day that last about 40-minutes and focused on behavioral or situational questions. Was recommended to on-site interview, where expenses were generously taken care of (flight, hotel, and transportation with an additionally $100 debit card to cover misc. expenses).
The on-site interview was rather laid-back overall. Arrived to the hotel with enough time to take a short nap before process. Starting at about 6PM, we gave a presentation on a segment with group members (who we had been previously in contact by Skype), dinner, had a long presentation by the recruiters about each segment, did a quick team-building exercise (building a catapult), and finished up for the night at a decent hour. We were done by 1AM, which gave us plenty of time to sleep before meeting at 7:30. It's hard to keep your attention on on the presentation after awhile but it's important to pay attention. Different segment have difference strength and helps you decide which segment you'd prefer.
The 2nd day was spent mostly traveling on the bus. Sadly, no field site was available for an interview so the recruiters improvised (this wasn't a big detriment to me as I had previous experience on drill rigs). Then there was a roundtable discussion with past or current field engineers, which was basically anyone relevant that they could round in up about 5 minutes at the office. This was a great time to ask frank questions, as the recruiters left the room during the discussion.
Returned early to the hotel and had to give a short presentation to the group about whether you'd like to work as a field engineer, what segment and way, and your biggest worry (biggest worry is almost always adapting to the lifestyle). This was ended with the shortest exit interview in history (under 2 minutes) with the recruiters that basically was for any last minute question you might have, and then met up with the recruiters for a nice dinner.
The recruiters overall were very laid-back (especially near the end of their recruiting season). While it was hard to be sociable with them directly, interacting with your other interviewees was very important. (Don't treat them as competition but as potential co-workers!)