You have to perform a presentation for the lab group of your work, be prepared for pertinent questions. Followed by time slots for one-to-one interviews with the PI, staff scientist, postdocs or other group members.

I contacted the PI I was interested in directly. This appears to be a good strategy and have since advised others to do so as well, with success. I found that it really helped that I had made contacts within NIH and had a reference from one of them. The interview process was pretty informal and conversational, fit/personality seemed very important as did general clear and logical thinking, specific skill sets were emphasized but not crucial, though some positions may be very focused on technical skills.General questions included the usual experience questions and "where do you see yourself in 5 years", "whats the ultimate dream" etc. Had to do a presentation of my doctoral research.