I'll admit, Marat/Sade is one of the most complex plays ever written, but it is a dream for any actor! I'm currently playing the Marquis de Sade in the play, and i've never been so challeneged in all my life! For the majority of the roles, not only do you need to play the character, you need to play the inmate of a lunatic asylum playing the character! For example, the inmate playing Jean-Paul Marat suffers from Paranoia, which must be taken into consideration during characterisation.
If you want to get the essence of the play, watch the DVD of Peter Brook's highly acclaimed RSC production. However, the play is so personal and powerful, the DVD does it no justice. Although the english translation still holds the power and Artaudian beauty (if that's a word!), it's not a patch on the German version. Trust me, i've seen extracts from the original production directed by Weiss himself - it's sheer brilliance!
To sum up - Marat/Sade has everything you could ask for. Whippings, Sex Maniacs, Aristocrats, Lunatics and even uses Commedia Dell'Arte techniques for the Four Singers! Artaud would have been very proud...