I think it is good that everyone has there own opinion on how Jill should look. I only hope that the casting director will pick up on the prospective Jill's acting quality as well! So far its been fab, the quality of acting and the physical appearences of actresses/tors have both great.
I'm thinking brown hair, shoulder length? Blue/grey eyes, tall maybe? and like some others have said, pretty but not too pretty? perhaps a feature that is unusual like a beauty spot or something like that that will set Jill appart from others. I mean like Anna Popplewell's full lips, Georgie Henly's smile etc...
I think that also the actress should be little known or a newcommer. Maybe someone like Sophie Turner from the HBO sieries Game of thrones that will be airing in america soon. (But be warned if you google her, there is an australian undies model with the same name...as i descovered
) She isn't like how I immediatly imagined Jill but her acting is amazing and she is deffinately tall and pretty!
But everyone has there own opinion and this is just one of many i'm sure
Whoever gets the role is bound to win everyone over in the end though,eh?