My involvement was the preparation of the actor scripts, the organisation of the actor schedule and assisting the voice director during the sessions.
I did a similar role on both In Cold Blood and El Dorado.
The best part is always the actual recording, particularly when you get to meet actors of real quality. When the sessions are going well, it's always great fun. There was one point when we were recording some interactions between George and Nico where everyone cracked up for a solid ten minutes because the lines out of context could be taken the wrong way. And were.
The hardest is worrying what the actors will make of the scripts.