I've got it! I've got her, rather...the perfect Marian. Are you ready? Minnie Driver.
She's still in her early thirties, but she can look older (and often does). She's certainly not beautiful; in fact, at times she can look, well, potato-faced. But she can spiff herself up when she needs to, so she can go in either direction -- something like Vivien Merchant, who could look dowdy or regal, whatever the role called for. And Driver is one hell of an actor. I've never seen her in a role that calls for the kind of authority a police lieutenant needs, but there's no doubt in my mind that she could do it.
What do you all think? Barbara?