I have seen videos where you created a domain model using ActiveRecord in real-time... elaborate on when and how you move to using full Hibernate in more detail.
The problem with presentations such as the one described in the question is that they are cheating. Oh, I really do work things on the fly, but it is like asking me to type with my eyes closed (as I am doing right now, just to see if I can).
I can do that, but it doesn’t really say much about my typing skills. The type of models that I create live tend to be very simple ones, things that I have done for dozens and hundreds of times. Building a model live on stage looks impressive, especially since I insist of the people in the audience picking the model. But it really isn’t that hard.
That said, the actual question is more interesting, what should you choose? Active Record, Fluent NHibernate or mapping files?
I, personally, like using mapping files, they are simple, uncomplicated and expose everything that NHibernate can do. Active Record is nice if you want to put the persistence definition right there along the class definition. Another major advantage of Active Record is that it is working hard to infer things for you. It makes working with it very easy.
Fluent NHibernate is mapping using C#. I don’t quite see the point there, especially since in some things FN decided to diverge from the NHibernate terminology, but I get why people love it. The part about it that I simply adore is the auto mapping support. That is a great way of getting things started.
But usually, even if I am using Fluent NHibernate or Active Record, I am mostly using them as scaffolding. At some point, I’ll ask them to generate the HBM for me and start working with them.
That is me, however, your mileage might vary.