Supposedly, she's not as interested in working as some of her contemporaries, and though she admits to surgical enhancement over the years, she looks very much like a real person, compared to the perhaps much more ambitious Faye Dunaway, who looks less and less recognizably human as time goes on.
Warren Beatty called her "the most beautiful and at the same time the most nervous person" he ever knew, which is an interesting declaration of neurosis from an actor who has famously aggravated directors with his arguably pathological insistence on "getting it right".
Christie's best roles exploit the tension between her surface capriciousness and inner reserves of empathic gravitas. This quality was maybe most evident in her earliest roles (see Billy Liar, Darling, Petulia), but it took on more interesting shades of pathos later, when her features started to exude melancholy.
The tragic undercurrents of Away From Her derive primarily from the popular image of Christie's hubristic youth, which superimposes itself over her middle-aged face and her character's struggle with space and time in ill-fitting, dichotomous flourishes.