At first, I thought it was that I didn't know what my character would do. That surprised me, because it's a character I know well. Eventually, I wrote the scene, but I wasn't happy with it. It didn't feel right, and I couldn't figure out why.
Then one day, in the shower, I had an epiphany. (And this is where I grumble about droughts that necessitate four minute showers, because I used to do some of my best thinking in the shower. These days epiphanies are fewer because I'm under the water a lot less.) The reason the scene wasn't working was that I had my character doing what the plot needed him to do, not what he would really do. I had forced his hand. In my head, where he ended up was where I'd envisaged him ending up right from the start. And the truth is -- he didn't want to go there. That's why I had so much trouble in the first place. It wasn't that I didn't know what he would do -- I did know, but it wasn't what I (or rather the plot) needed him to do.
Okay, easy enough. Scrap the second half of the scene and rewrite it. The more I looked at the scene, though, the more I liked what I had done, the emotional journey he'd gone on, the reasoning he'd used. So I put the scene aside again. I reread it. And reread it.
Then I thought that maybe I could just cut and paste some of the old scene and rewrite a new bit in the middle. I did this and now suspect I've ended up with a mishmash of what the scene had been and what it should be, but I have something I can work on now and refine. I'm excited. I love the refining process. I love writing! I'm going back to it right now.