Applied online initially after conversation with a campus recruiter. Was contacted for an initial HR phone interview about a month later which was then followed by a technical phone interview conducted by a research staff member. Following the phone interviews, I flew out to interview in person. Due to weather, I had to reschedule to the next day. IDA went to great lengths to rebook my travel and hotel and reschedule my interviews at the last minute. Props to them.
The in person interview consisted of a research presentation along with a technical panel interview, HR talk, 1:1s with staff, lunch, and a meeting with the division director. I would say the atmosphere was conversational rather than trying to trip me up. The staff mainly wanted to learn about my background and interests and answer any questions I had about work at IDA. Similar to Google, it's much more about how you think than what knowledge you are carrying around with you at the moment.