The doctors are in (town)

Chicagoans are used to spotting bits of the Windy City on NBC’s hit drama “ER,” which kicks off its fourth season with a one-hour live broadcast at 9 p.m. Thursday on Channel 5. But when they spot the actors walking around their streets, they get a little rabid.

For instance, one woman asked George Clooney – who plays womanizing pediatrician Doug Ross – to sign her baby’s Pampers.

“This kind of thing happens all the time,” said publicist Andrew Shipps, who was in town with the cast of “ER” two weeks ago to film exterior shots for the show. “And George always does it. He always takes the time to give them what they want.”

While filmed primarily in Burbank, Calif., “ER” is set in Chicago in the fictional County General Memorial Hospital. The cast flies to Chicago four times each season to film enough footage for six episodes each trip.

Certain settings, such as the hospital’s ambulance bay and the seedy diner Doc Magoos that the ER staff members favor, obviously are faux Chicago elements. However, shots of the Chicago River, the skyline and Michigan Avenue are the real deal.

On this particular Sunday, Clooney and Julianna Margulies, who plays head nurse Carol Hathaway, were on a downtown L platform at Chicago and Franklin. This graffiti-laced platform is familiar to “ER” fans, as well as regular viewers of CBS’ “Chicago Hope.” It has been used in many scenes, including one episode last season when Dr. Carter (Noah Wyle) and Dr. Benton (Eriq LaSalle) talk about the suicide of a colleague.

Clooney and Margulies would be on this platform for the next couple of hours filming a scene that will air for just a few minutes on the show’s third episode Oct. 9. Last season, former lovers Ross and Hathaway left viewers wondering whether the couple would rekindle their relationship. This scene dealt with the aftermath of their passionate kiss.

After two hours of watching the photogenic actors go through their scenes, a visitor on the set had no problem parroting their lines. But in fairness to the producers who would like to keep the plot element a secret until this episode airs next month, we’ll just repeat one line from each actor:

“I’ve prepared a very elaborate argument,” says Ross.

“I already had the argument with myself,” Hathaway replies.

OK, here’s one more hint: The characters have a physical encounter. But we’re not blabbing whether it’s a kiss or a slap in the face.

Wearing a white windbreaker, blue T-shirt and gray pants, Clooney clowned around between takes. Chomping on a stick of gum, he animatedly talked about bursting capillaries one moment and then flapped his arms wildly, pretending to be falling onto the tracks below. Then he manipulated the gum over his front teeth and pretended to be wearing a bad set of dentures.

But when it came time for the cameras to roll, Clooney turned serious and repeated his lines to himself. By adding subtle nuances to different words, he changed the context of the dialogue from inquisitive to sheepish. For the first take, he settled for sheepish.

While friendly with the crew, Margulies was more subdued and introspective than her co-star. Dressed in a gray T-shirt, blue jeans and burgundy Hush Puppies that matched her leather blazer, the pencil-thin actress twisted her curly black hair around her fingers in between scenes and remained focused throughout filming.

But when, after the third take, Clooney grabbed her and joked, “Come here. Let me crack your head,” Margulies cracked up.

“George is always joking around,” said Barbara Purros, the 5-foot-5 stand-in for Margulies. “They’re all very professional and nice, but George always keeps things light.”

“It’s a pleasure working with the people on `ER,’ ” said Clooney’s stand-in, 6-foot-tall Greg Mashburn. “As an actor, I always feel like I’m getting paid to learn.”

Mashburn got his Screen Actors Guild card working on “ER.” While Clooney was off filming a movie, a producer had Mashburn respond to something Hathaway was saying to Ross.

“Julianna was on (David) Letterman’s show and she was telling him about an extra who got his SAG card because of `ER,’ ” said Mashburn, who also is the stand-in for Wyle and Anthony Edwards (who plays Dr. Mark Greene), as well as Kyle Chandler on “Early Edition,” a show that is shot entirely in Chicago. “That was kind of exciting.”

There were about three dozen other extras working that afternoon. They were scattered on the platform, dressed in business attire, athletic wear and student gear. Others were on the L train, reading newspapers and peering out the windows.

When one of the extras – all of whom were supposed to remain silent – was overheard talking during a scene as she walked past Clooney and Margulies, who were delivering their lines, she was ordered off the set.

Makeup artist Jamie Weiss was on hand with her bag of tricks to touch up the actors. But they didn’t need much tweaking. Margulies got the occasional dab of powder. Clooney was left alone.

“He has really good skin,” said Weiss, whose brother Michael T. Weiss stars on NBC’s “The Pretender.” “We do as little to him as possible.”

By early evening, the cast was dismissed for their dinner break before heading over to the Old Irving Park neighborhood to shoot exteriors of Hathaway’s house.

“We’ll be working until at least midnight,” Clooney lamented.

“We can’t shoot the next scene until it gets dark.”

But their work wasn’t done just yet. Before they could make it down the stairs of the L platform, the two actors were besieged by autograph seekers, as well as extras wanting to shake their hands.

A group of children clasping shreds of paper got first priority from Clooney, who asked for their names and personalized autographs for them. The group of young women waiting for him on the street by the doughnut wagon was next. And even the man waiting for Clooney outside his trailer was treated with respect, even after he thrust six photos and a book at the actor to sign.

“See, I told you this happens all the time,” Shipps said. “It happens everywhere.