With her exotically pale looks and her taste for experimental theater and film, Tilda Swinton is an unlikely choice to play an American mother in the expertly taut melodrama The Deep End. Previously, the 42-year-old Scottish actress was best known for the gender-morphing title character she played in Sally Potter’s arty 1993 adaptation of Virginia Woolf’s Orlando. Yet that same otherworldly intensity and lunar beauty is what makes Swinton’s performance as Margaret so arresting, and so appropriate to the story of a woman willing to dive in over her head to protect her teenage son from a blackmail threat. Swinton commands absolute attention not only when Margaret is doing something utterly film-noirish — such as hiding a body — but also when she’s engaged in such utterly apple-pie-normal mom-ish activity as talking to the local banker or driving along scenic, all-American Lake Tahoe roads. And when the briskly competent Margaret warms under the soulful attentions of Goran Visnjic (as a blackmailer’s hench- man), Swinton is killer in her ability to convey clashing maternal and erotic desires. Margaret, a quintessential modern do-it-all dame to begin with, is all the more fabulous a film-noir heroine for Swinton’s hot cool.