The actors not only are in rehearsal for close to eight hours a day, they are also expected to learn their lines when they're not rehearsing, and are expected to have them memorized by the third if not the second week. (The fight scene I mentioned earlier was choreographed outside of the normal rehearsal hours, meaning the actors involved had to show up early that day.)
However, don't be looking for anybody giving away any acting tips or hints on how to mount your own production of Hamlet. In fact I had forgotten how frustrating it can be to talk to actors and directors about their process for developing a character or staging a play. It's not that they don't know what they're doing, it's just not the sort of thing you can easily articulate to people who are not directly involved with the project you're working on. While the woman (Dearbhla Molloy) playing the role of Hamlet's mother, Gertrude, says something about drawing upon her relationship with her son to help her prepare for the role, that's the closest any of the actors come to talking specifics about what they did to help them prepare. Even when we overhear the rehearsals via the camera, it doesn't make much of a difference as everybody seems to be talking in a shorthand incomprehensible to those who don't work in theatre. At one point we watch Jacobi giving notes to his actors—telling them things they need to work on to improve their performance—and while his words obviously mean a lot to his actors, the fact that he's telling them they need to listen to each other more instead of anticipating their lines will probably mean nothing to those who haven't worked in theatre in some capacity.
The other thing you have to be aware of is that even when the camera does capture some of Branagh's, or any character's, performance, it will seem like they are overacting horribly. This is when you realize the huge difference between film and stage acting. Aside from having to memorize the whole script at once instead of merely whatever pages you'll be shooting on a given day, actors are also having to make themselves understood by people who are as much as 200 feet away from them without using any amplification when they are on stage. On film they will look ridiculous because of the medium's tendency to exaggerate even the smallest motion. (In the interview conducted years later with Jacobi, the director of the movie asks him what he thinks is the biggest challenge facing the classical theatre today. Jacobi's answer is actors have become so reliant on amplification few know how to use their voices sufficiently well to handle the nuances required to perform Shakespeare live anymore.)