In case somebody is interested:
I finished my alum interview on Dec 21st.
It was a great experience. The guy was already absolutely nice on the phone, picked me up at the station and we then talked at a cafe in a very casual environment.
The interview took almost 2h, but I am sure this is no indication for other interviews. He took the first 30mins alone to show me pictures of Tuck and his classmates as well as the professors. He also explained the campus to me and how people live. As I havent been to Tuck yet, I got a great impression of how life would be like there.
We then moved on to my past experience, but in general it was more of a conversation rather than an interview. After about 1h30min the official part was pretty much over and we spoke about what he did in his company etc.
All in all a great interview experience, showing that Tuck people are as friendly as they say.