First some recruiters came by campus at my university where I gave a 20-30 minute presentation on my research. After this initial interaction, I was invited for an on-site interview in Rahway, NJ. The day before, I met with some new scientists who have been working with Merck for about a year for dinner. This was very relaxed and casual.
The following morning consisted of a 45-50 minute presentation of my research followed by Q&A. Everything they asked was very positive and thoughtful. The rest of the day was filled with seven 45-minute one-on-one "interviews". I use quotations because it did not feel like interviews; it was just more casual conversations talking about research and what it's like to work at Merck. Make sure you have questions prepared even if you have to repeat it to different people. You can also ask them to talk about their research projects.
Even though I had a very positive interview experience at Merck, they ultimately decided not to give me an offer because I did not have post-doctoral experience. I was surprised by this because everyone gave me positive feedback. I guess if you're a Ph.D. student competing against Ph.D. + Post-doc candidates, it puts you at a disadvantage instantly.