Pat, by that reasoning, you shouldn't write the second half of a movie
until you've filmed the first half, which is silly.

A story isn't about the nuances an actor brings to a performance, the
story is about the *story*, and the characters' role in that story. One of
the biggest areas of misinformation in the public consciousness is how much
imput actors and directors have on characters and stories in TV. If you
don't know what it is you want to say in your story, and want to wait for the
actor's choices in determining how to read your lines before you figure out
what story you're telling, you're in serious trouble.

It comes down to, Are you just filling out time, being arbitrary, or do
you have something to *say* in your story? If the former, then sure, be
arbitrary, be kicked around on the winds of public opinion, let the actor's
sudden decision to smile instead of frown in a scene dictate your story. Me,
I'd rather tell a story, and risk failing a few times, than abrogating my
responsibility as a storyteller and a writer.

jms