I went through Palantir's interview process twice (once for internship and once for full-time). Both times it was a full-day process, and I was exhausted by the end of it. There was 4 or 5 interviews for internship (and I had to talk to one of the founders which I think might have been another interview?), and similarly for full-time (didn't talk to founder this time.)
Each interview is usually 5 minutes of small talk / background question (I doubt they care) + 40 minutes of coding/product/algorithms question. The questions were not too easy, but things one is expected to know for the job (especially for my full-time interviews, maybe they toned down the questions?).
My interviewers were very nice. One even remembered some facts I told him during the interview about a few days later. At the same time, I could tell that they all had something more important to do. There was a lot of rushing in and out (even with the recruiting coordinator). During the day I was rushed from buildings to lunch to buildings. My takeaway of the day was that everyone is just every intense at Palantir. I wasn't affected by it much, but I can see how a brilliant but nervous person can mess up their interviews because of the atmosphere.
To future applicants: Expect a lot of rushing, so take deep breaths whenever you can. Palantir is a very startup-y start-up, so brace yourself for the interview day, and use it as a opportunity to see if it's a fit for you.