Mothering Heights

But it's hard to out-glow Julia Roberts. ''She smiles,'' says Sarandon, ''and the sun comes out.'' Half the free world noticed that eight years ago, when she smiled in Pretty Woman. Since then, Roberts, who won her second Oscar nomination (her first was for supporting actress in 1989's Steel Magnolias) for that career-making role, has sometimes seemed on a crusade to prove she's more than a pretty face (see the 1996 double smilectomy of Michael Collins and Mary Reilly). But in '97, Roberts began rechanneling her Olympian charm. After taking on Mel Gibson in Conspiracy Theory and Cameron Diaz in My Best Friend's Wedding, she can be her halogen self and still go claw to claw with one of today's most respected actresses.

''If I'm better because I'm with Susan...then so be it,'' says Roberts, who'll next reunite with Pretty Woman costar Richard Gere in Runaway Bride. ''So I guess I'm not really that bad...'' — she takes a deep breath — ''...'cause I was smart enough to find the thing with the girl to make me better.''

''I remember the first big rehearsal day with Susan,'' Roberts says. ''Suddenly, I thought, What if she thinks I suck? That was the first time it occurred to me, Gee, this could be bad.''

Sarandon, who flourished in an equally complex working relationship with her companion, Dead Man Walking director Tim Robbins, says Roberts never had to worry. ''If she'd been unprofessional, or unable to deliver, there'd have been a problem.'' Still, professionalism didn't stop her from giving Roberts frequent tongue-lashings — fortunately, part of the script. Stepmom has more catfighting than a Jerry Springer marathon.

''It seemed like banter when we were revising dialogue,'' Roberts recalls. ''But in rehearsal, it quickly became Susan as the aggressor. I think she absolutely enjoyed herself.'' Sarandon brightens at the memory. ''I kind of never get a chance to be nasty, so I was really happy to be mean and bitchy.''

The actresses were so good at pretending to hate each other, they wound up wondering if they actually did. ''I remember thinking, Why do I feel my life is spinning out of control?'' Roberts says. ''Then I'd realize: Oh, it's because I've been s--- on all day.'' Nor was it easy for Sarandon, who had to wear frumpy caftans and terminal makeup while her costar enjoyed a superfly wardrobe and killer hair. Didn't Sarandon ever want to strangle Roberts, a little? ''Let's put it this way,'' she says. ''It's never great for your ego when you finish getting made up and come out looking worse than when you came in.''

In the end, nobody wound up in therapy. ''When any movie goes well, it's a miracle,'' Roberts says. ''It's only when you experience the antithesis that you worship the miracle of harmony. It really was a seamless experience — except for the title.''

''You come in, you do it, you leave,'' says Sarandon. ''It's just a movie.''

We believe them...but that could just be us. They are, after all, great actresses.