Lest you get the impression Sugar is the cliched "hooker with the heart of gold", it only takes remembering how carefully Garai's character orchestrated everything to make herself indispensable to Rackham. However, we do see that while she doesn't have much respect for her clients, in fact she dreams of taking murderous revenge on most of them, including Rackham, we also see her compassion for those who she sees as being mistreated by the world. In her relationship with Mrs. Rackham, Garai does a remarkable job of being completely sincere in her feelings of pity for the other woman, while a part of her obviously would prefer if she were to just vanish. There is a blade of steel inside of her from having lived in the survival of the fittest streets of London, and while she may be sympathetic to others, we have the feeling that she's not going to let anybody get in her way of her dream of a new life.
Of course she also recognizes the feelings of being caged that Mrs. Rackham suffers from as being identical to how she felt about her old life. However, as Hale so magnificently shows, Mrs. Rackham's prison is caused by the pressures and expectations of society on her to behave in a certain manner. Hale manages to walk the line between overacting and playing somebody suffering from delusions and extreme nervousness wonderfully. It would have been easy to play this type of character as a single note, in a constant state of hysteria. However she makes her a far more believable character by showing us glimpses of the person she had been before she became afflicted by her illness. This is important because if we didn't see anything redeemable in her, Rackham's love for her wouldn't have been believable.
In the bonus features included on the second of the two discs in this package, we hear from both the actors and the technical people about how they approached their job on this shoot. While nobody goes into tremendous detail, the production designers and cinematographer do explain the techniques they used and the effects they were trying to achieve. In their interviews both Garai and O'Dowd explain the approaches they took to try and humanize their characters. I would have liked to hear more of how O'Dowd, whose background is mainly comedy, might have changed his approach for this role from what he's done in the past, but he just talked about how he tried to inject some humour into his character.