It was dinner as usual last Tuesday at Parc on Rittenhouse Square. The crowd at the bar was chattering mightily into the tin ceiling. A slender blond woman wearing a long-sleeve white-and-green knit and ripped jeans walked in with an older blond woman in a black dress.

Isn't that . . .?

Yes, Reese Witherspoon, and her mother, Betty.

Such scenes are playing out all over town lately: The Legally Blonde star with beau Jake Gyllenhaal shopping at Whole Foods in Wynnewood. Tucking into carne asada at Alma de Cuba on Walnut Street. Cuddling at Marathon Grill in West Philadelphia. Lunching with her kids at Ruby's Diner in Suburban Square in Ardmore.

Actors Paul Rudd and Owen Wilson joining the late-night crowd at Continental Mid-town on Chestnut Street. Jack Nicholson ducking out of Parc for a smoke.

If Philadelphia seems star-heavy this summer, the reason is that a month ago, writer-director James L. Brooks began shooting a still-untitled comedy here, mostly in University City and South Philadelphia. Witherspoon plays a softball player torn between the affections of Wilson and Rudd. Philadelphia, which offered Pennsylvania tax incentives to draw the Columbia Pictures film, is standing in for Washington.

The director, producers, main cast, and crew members will be living in the area till at least mid-October. Other actors pop in and out. Nicholson was here last month and is expected to be back in mid-September for the duration.

Meanwhile, the summer has been a celeb-watcher's dream, and the stars have been free to come and go as they please.

How to tell the stars in a crowded restaurant? They're the ones without cell-phone cameras in hand or communicating the sightings on Twitter.

Wilson regularly rides a bike outside the Armory on Drexel University's campus, where most of the interiors are being filmed. He also turned up at the Sporting Club at the Bellevue to play squash, and soaked up culture at the Art Museum.

Rudd got a visit last week from Jenkintown-reared buddy Bradley Cooper and Cooper's flame, Renee Zellweger, causing baristas at the Washington Square Starbucks to stop in mid-latte.

As A-listers, Witherspoon and Gyllenhaal are routinely tracked by paparazzi, but there's not been much of a frenzy here aside from a freelancer shooting for the INF agency. Two photographers from the D.C. area became so bored while searching for Witherhaal/Gyllenspoon last month that they sought out a photo op with singer Marilyn Manson in Old City.

Wilson and Rudd, who are staying in Center City, seem not to have been too annoyed by picture-takers, but Witherspoon, renting a gated house on the Main Line, is more guarded.

Last week, HughE Dillon, a Center City paralegal by day and paparazzo by night who blogs the social scene at PhillyChitChat.com, said Witherspoon slipped out the back door of Morimoto on Chestnut Street and hired a cab to take her and Gyllenhaal around the block to their SUV parked nearby.

Earlier this month, the couple left the Elton John-Billy Joel show in their SUV. Gyllenhaal, who was driving, apparently noticed that he was being followed. It was a sedan occupied by the Washington photogs, who go by the nom de guerre Brandon Todd. They said Gyllenhaal gunned the SUV through two lights and zipped onto the Schuylkill Expressway.

Sharon Pinkenson, who heads the Greater Philadelphia Film Office, said the Brooks film would create a $100 million economic impact.

About half that, one might argue, on restaurant tabs.

Contact staff writer Michael Klein at 215-854-5514 or mklein@phillynews.com.