There's no reason I should not want to write this scene, and no excuse for it to be dull -- my two favorite characters having an emotionally charged conversation which proves to be pivotal both to their relationship and to the course of the book. And yet, to borrow an expression from jenny_moss, all I could think when I sat down to work on it was "Wah, wah, wah."
So, since none of my usual writing techniques seemed to be working (and taking a few days off from the story didn't seem to be working either), I resorted to trying other authors' techniques.
This afternoon I tried Laini Taylor's trick of starting a "Working Document" and just writing down any and every thought that came into my head about the scene, including my own insecurities about writing it, but it didn't seem to shake anything loose. Then tonight I tried pameladean's old method of writing the scene as a play first and then going back and putting in the narrative later.
Interestingly enough, that latter one did help. I realized that I'd had a lot of internal monologue and description in the scene which was causing me to lose the flow of the conversation, and that when I stripped that away I could see much more readily what my characters' emotions and reactions ought to be. I'm not sure it's a tool I'll use often, but in this case it seems to be the right one, because I've managed to get Faraday and Alison talking again -- so thanks, pameladean!