Julia Roberts gives some wonderful readings, particularly comic ones, when, for instance, she tells Larry the name of Dan's novel. And her expressions can be quite specifically eloquent, her lowered gaze during the break-up fight with Larry, for instance, in answer to his saying, "But we're happy, aren't we?" which he had intended as a rhetorical question.
And Roberts isn't afraid of all that's unflattering about Anna, but she lacks the theatrical technique to draw us in to the character's waffling, which hurts other people worse than decisiveness would and doesn't even get her what she wants. Roberts also gets the hollow self-loathing of Anna's "I'm disgusting," spoken immediately after confessing how long she's been hiding her adultery. But Roberts never really seems disgusting here, or whorish or as enamored of a guilty fuck as Larry says she is, or as witchy as Alice says she is, and we can't be sure whether Marber intends the other characters to be misspeaking or exaggerating (doubtful) or whether Roberts is simply not right for the play. She's not wrong for it, but that isn't enough. (Probably a bigger problem for the movie's success is that when Roberts runs her starshine through a dark filter, it gets a little dim. Not as dim here as in Mary Reilly (1996), which was inferior material, but still.)
Alice, for whom waitressing is not a temporary thing as Anna assumes, is the most masochistic role, just as Larry is the most sadistic, and she reminds you of any number of underappreciated nice girls in movies. Natalie Portman stands with the very best of them, Juliette Binoche in The Unbearable Lightness of Being (1988), for example. Her performance even benefits from a certain gaucheness in her domestic scenes, which involve the most direct emoting in the movie. She gives Alice enough teary-babyishness to impress you with the amount of shellac she has applied over it in the public scenes. The inscrutably pert way she says "Thank you" over and over to Larry in the Paradise Room, for instance, is classic, so repetitive as to be maddening and yet beautifully expressive, as measured by what Alice is refusing to respond to.
Portman has been directed very well so that the waif-dominatrix iciness Alice gives off registers as deliberate protective cover. When your life is out of kilter, or you just want money, you give the world what it expects and values, but you never identify success in the assumed role with satisfaction. This leaves Alice open to being misinterpreted and doesn't help her get what she wants anymore than Anna does, despite being less compromised than her rival. She's the last character we see, now turning heads as she bobs down a mobbed Times Square sidewalk in the same seductive slow motion as in the opening, on her own again.