The Screenplay directive will uses parts delayed processing to inject a transform which takes a raw screenplay as specified on the web and format it into nodes and node classes which can then be styled after various writer transforms.
The directive might also roles which can later be interpreted by the transform, such as parentheticals.
I don't think a "screenplay" directive will fit nicely into "standard" reStructuredText.
It would better be implemented as an extension.
If an extension is OK for you and you still plan to write such a directive, we can help.
Just ask on the docutils-develop list or re-open this ticket.