Nicole Kidman had been rumored as another possible addition to the cast, but she has more regal obligations and cannot join, according to THR. Kidman, who played the lead role in von Trier's Dogville is currently busy playing Grace Kelly in Olivier Dahan's Grace of Monaco.
Dafoe, meanwhile, will play a supporting role in the two-part erotic Nymphomaniac, which will be released in both a hardcore and soft core version. Dafoe is currently in Hamburg, starring in Anton Corbijn's drama A Most Wanted Man.
Von Trier veteran Udo Kier, who has taken part in a number of von Trier's films including his most recent Melancholia has also joined the cast along with French actor Jean-Marc Barr (Dogville), Caroline Goodall (Schindler's List), Kate Ashfield (Shaun of the Dead), Saskia Reeves (Butterfly Kiss) and Danish actor Omar Shargawi (R).
In the film, Gainsbourg stars as Jo, a self-described nymphomaniac who reveals her story to an older bachelor played by Stellan Skarsgard.
Rumors have flown over the summer whether named actors would take part in sexually explicit scenes, though porn-star "stand-ins" have also been rumored to be engaged on camera. Shia LaBeouf fueled the rumor when he said he was ready to "go all the way" in Nymphomaniac.
[Source: THR]
Source:
http://movieline.com/2012/10/17/willem-dafoe-joins-lars-von-triers-nymphomaniac-nicole-kidman-heads-out/