The first half of this exchange was included in when the (quite good, revealing) interview was published, in November, but I haven't seen it quoted very much (though the use of a double for the genital-mutilation scenes had been reported elsewhere). But the unedited transcript Keogh has posted is well worth musing over:
PK: William Dafoe, and I think you've mentioned this in another interview, is probably the worst therapist in the history of movies. How would you advise him to treat the Charlotte Gainsbourgh character, and what does he do wrong?
Lars Von Trier: Yeah, first of all, I have been undergoing this cognitive therapy for three years, and it's I think it's quite typical for me to be sarcastic. You can say that one of the main ideas behind any treatment of this also is that a fear is a thought, and, you know, it doesn't change reality. You can say in the film it's changed reality. All that was kind of what you could read up about the film. I wouldn't let him treat her in any other way than with his dick, he has an enormous dick, but that maybe I took also...he's extremely well-equipped. And we had to kind of take the scenes out of the film, we had a stand-in for him, we had to take the scenes out with his own dick.
PK: Hold on —-You had a stand-in dick? You had to have a stand in dick for Dafoe?
LV: Yes, yes, we had to have, because Will's own was too big.
PK: Too big to fit in the screen?
LV: (laughs) No, too big because everybody got very confused when they saw it.
PK: People would get intimidated. Especially when he starts-
LV: Especially when he-
PK: When he ejaculates blood, that was uh-
LV: Oh yeah, yeah. That was the double.
PK: It's quite a trick.
LV: Uh, yes.
Thomas Vinterberg, who in 2005 directed von Trier's script Dear Wendy, spoke frequently at the time of the film's release about von Trier's feelings about dicks. Here's a representative quote (which also explains one of the reasons why Dear Wendy is a bad, presumptuous movie:
In Dear Wendy, I think primarily it is about Lars and the white man feeling inferior to the black male... Sebastian, the black boy in the film, is a man of action. Lars is always talking about black men's genitals and the size of them and he is deeply fascinated and envious about it...
Lars von Trier cannot stop talking about cock.
Anyway, does Willem Dafoe have a confusingly large penis? Let's ask the man himself...
Well, that settles that, then.