I was invited onsite and had 2 back-to-back interviews, lunch (with a staff member), and then two more back-to-back interviews. Every interview was a one-on-one standard one-hour whiteboard interview. We started with a brief overview of my interests and resume, and got to the question about 15 minutes in, with about 10 minutes at the end of each interview for me to ask them questions. The interviewers valued enthusiasm and critical thinking/communication, as I am sure I didn't get two of the 4 interviewers' problems right. I was surprised by how explicitly they wanted me to write code on the whiteboard (often pointing out minor syntax errors and making me write code to solve edge cases rather than just pointing them out). One interviewer wanted me to write in C despite me professing little recent experience in it and my fear that it would lead to too much writing for a timed interview (the whiteboard was filled entirely with code at the end of each question).