First there was a phone interview that lasted approximately 1 hour. Within a few weeks, they flew me out for an on-site interview. I had to pay out of pocket to book my hotel and flight, but was later reimbursed. The onsite consisted of meeting with the PI I'd work for, a lab/facilities tour, a one hour presentation by me to the group, then several one-on-one meetings with the director of the center and various other people. Lunch was in a conference room where other postdocs gave me presentations of their research and I asked them questions. There wasn't really any one-on-one time with other postdocs, as the PI was in the lunch room with us. I wish I would have had some one-on-one time with the other postdocs so I could have asked questions about the PI's management style.
During the portion of the interview with the director, I was offered the position on the spot. I verbally accepted. The official letter came later.
Surprisingly, the hardest part of the process has been the onboarding. It's a dual appointment with BWH, and BWH has been extremely rude and unhelpful throughout the process. I feel as if I didn't press issues on my end (like setting up the occupational health screening, etc) then my start date would have been continually pushed back (it was already delayed by 1 week). The go-to person at BWH has been a nightmare to work with and has said things like "you are taking up too much of my time," "I will apologize on your behalf to HR for contacting them," "now you have wasted my time," etc., when I have e-mailed questions and waited for more than 5 days for a response. The whole onboarding process literally almost made me want to throw my hands up and look for a position elsewhere. It was the most unwelcoming thing I have ever experienced as a new hire!! I can only hope my interaction with this person ends after this stage and it's smooth sailing from here out.
The interview process was actually amazing. If you come prepared and knowledgeable about the lab you're joining, then there shouldn't be anything to worry about. I was not at all expecting to be offered the position on the spot. The only thing that affected my overall experience was dealing with this super negative person for my actual onboarding. (There were absolutely no problems with the center I'm joining or Harvard in general, just with BWH).