Big Dave made a comment on a recent post. I was replying to the comment when I realised I was rambling on a bit and that this should really be a post to itself.
So, here's the answer (in my rambling opinion):
I think the writer contributes about 80% of what the audience experiences in a play.
It's the director's job to ensure that what the writer intended is conveyed successfully to the audience.
Now, the director can make decisions about the look of the play (lighting; furniture; set; costumes) with his prduction staff, and can determine the pace it's played at. But the fundamental 'message' of the play is all the writer's.
Bad direction can easily ruin a good play, and a bad play can be improved somewhat by a good director. That said, if a director is putting his/her own agenda on top of the writer's intentions, they're not doing their job properly.
It's interesting that Dave uses the word 'vacant'. An unperformed play is simply an intention to make some point or other. It's not until the work is actually in rehearsal that it comes alive and hidden depths are found (sometimes, depths that the play's writer was unaware of).
The challenge, and the excitement, of bringing a play to the stage, is to make it accessible and exciting for an audience, without losing the playwright's underlying message - however deep or trivial that message might be!