NEW YORK — Ah, the live theater, where even great big movie stars can expect the unexpected.

At the opening of Broadway’s “Three Days of Rain,” for instance, Julia Roberts watched her co-star, Paul Rudd, throw a journal — the show’s most important prop– to the ground in a fit of anger — and then watched the book flip off the stage and into the aisle.

Like a pro, Roberts cracked nary a smile. But Rudd gave a nod of thanks to the nice gentleman in the second row who picked up the book and slipped it back onto the stage.

After the show, the woman of the evening did some quick gushing on the red carpet — she said she was “really, really happy,” you’ll be surprised to learn — and sailed into the party at Cipriani.

Rudd remembered going in for the initial reading of the play with Roberts, playwright Richard Greenberg and director Joe Mantello, only to learn he had prepared for the wrong part. “I burst into a cold sweat,” he said. (But the role he didn’t prep for is the one he’s playing on Broadway, so he must have done OK.)