Reason for stopping: I have a quandry.
I need this character onstage because I need to drop some more hints to what's going on before I start the big reveal. And I have some characterization stuff I need to do.
I just don't know what the actual plot purpose of the scene is, as opposed to its structural and characterization purposes. In other words, I haven't identified what changes during this particular scene. So I know what this is for, but I don't know what it does.
And until I know what it does, I'm not sure I can finish writing it beyond the setup.