Anyone who's seen The Exorcist has probably at one point or another looked up, or asked someone who knows, what Regan and Father Karras are saying to each other in the bedroom when she speaks Latin and French. I know I did. And when I did I found it not only interesting, but invaluable in assessing the character of Regan and/or Pazuzu, the demononic spirit that possesses her. Here's the dialogue:
Regan: Mirabile dictu, don’t you agree? Here she's saying that it's "miraculous to speak of" or "talk about" or "discuss," having an exorcism that is.
Karras is curious about her speaking Latin, asks if she does speak it and she says, "Ego te absolvo" which you could probably easily figure out on your own means "I absolve you" (duh) or more roughly, "I forgive you," that is, you're absolved of your sins.
Karras then asks, "Quod nomen mihi est?” or basically "tell me what my name is?"
Regan responds, "Bon Jour" which we all know means "Good Day" in French. Karras repeats his question in Latin and then Regan says, again in French, "La plume de ma tante,” which means, "The quill of my aunt." The quill there referring to the kind of pen dipped in an ink well.
When I looked up that phrase many moons ago, I discovered it is a common phrase in basic French textbooks. It is usually written as "la plume de ma tante est sur le bureau de mon oncle" or "the quill of my aunt in on the desk of my uncle."
Okay, so what? Why do I consider this invaluable? Because early on in the movie there is an open question as to whether Regan is truly possessed by a demonic spirit or simply schizophrenic. And this adds to those early layers of ambiguity. The spirit possessing Regan claims to be the Devil and yet can only muster up "good day" and "the quill of my aunt" in conversation because that's all Regan has studied, or remembered, in French class. It gives the viewer early on two possibilities: Either she really is schizophrenic or this is some minor pissant demon. And Karras doesn't appear to be buying it for a second. His question is perfectly constructed. He's saying, "Okay smartass, if you speak Latin, you'll say 'Karras' in response to this question." But Regan doesn't. She says "Bon Jour" because chances are, she doesn't know what Karras just asked her. It's brilliant and reveals an amateur in Regan, facing a superior opponent in Karras. She's fooled everyone else, but not our man Karras. And this adds another level of ambiguity and doubt for Karras himself, now believing even less that she is truly possessed.
Unfortunately, for me (though I know it's not with many fans of the movie), once she's levitated and twisted her head around the argument is settled, it's Pazuzu. A part of me has 
always wished William Blatty and William Friedkin hadn't put in the levitation, the head turn or the backwards talking because it removes all ambiguity. Even Karras seeing his mother on the bed, feeling Pazuzu enter into him at the end and hearing the homeless man from the subway in Regan's bedroom could arguably all be Karras. He's under emotional duress and none of that would be out of the question. But the physical act of levitation removes all doubt.
I'm not saying I don't want there to be demonic possession, just that I sometimes wish Blatty and Friedkin had offered nothing solid in the way of evidence, right up to the closing credits. Because honestly, I don't care if she's actually possessed. Whether she is or not does not diminish any of the anguish of Karras, especially given that he believes by the end. His belief in her possession is what matters, not the viewers. But that's a minor "what if" quibble for this classic and emotionally painful tale. And the bedroom scene adds another layer to the Regan/Karras confrontation near the end, knowing where they started and how Karras initially had the upper hand.
And now I bid you adieu to ponder their relationship further while I seek out the quill of my aunt, which, if I am not mistaken, is on the desk of my uncle. Bon jour.
