Auditorium 17 at the AMC-Loews at The Waterfront had no fancy VIP seats or IMAX screen but it had one thing the other theaters did not on Saturday night: dreamboat actor Taylor Lautner.

His famous abs were hidden under a white T-shirt, paired with jeans, but he produced shrieks of delight nevertheless among roughly 170 moviegoers waiting for the 8 p.m. showing of “The Twilight Saga: Eclipse.”

The film’s cast has been making surprise appearances around the country as a way to thank moviegoers and Mr. Lautner showed up in Pittsburgh, where he is making a film called “Abduction.”

If the fans didn’t know Mr. Lautner was coming — one cited “hearsay,” as if she were a courtroom attorney — they suspected something was up last night once youthful security guards in suits marched in and took positions near the screen on the floor and along the hallway leading into the theater.

Patrons whispered and tapped out text messages, saying they’d heard a surprise might be in store. Then AMC-Loews employee Patrick Fisher introduced Mr. Lautner, called out “Who’s ready?” and asked patrons to remain in their seats.

Mr. Lautner bounded to the front of the auditorium as cell phone cameras captured the moment for all eternity … or Facebook. Fans screamed, whooped and declared their love for the 18-year-old actor who plays werewolf Jacob Black in the wildly popular vampire franchise.

“How are you guys?” he asked, his remarks amplified by a microphone. Smiling and joking with the “awesome” audience, he laughed when someone urged him to “take your shirt off.”

Spotting three women in “Twilight”-themed shirts at the end of the third row of seats, he told them “I love the shirts,” then corrected himself by saying, “I like the one at the end” — a reference to its “Jacob’s Army” proclamation.

The wearer of that garment, Kara Bussard, of Moon, was celebrating her upcoming 16th birthday with her sister, Erin Dargo, 29, of Brentwood. They’d initially purchased tickets for the 10 p.m. screening, Kara said, but went back to buy seats for the earlier show after hearing a rumor about Mr. Lautner’s appearance from a worker at the Aeropostale store in the Waterfront.

When Ms. Dargo yelled, “It’s her birthday” and pointed to her sister, Mr. Lautner wished Kara a happy birthday.

“I’m so excited I can’t think straight,” Kara gushed later. “I could cry.”

Mr. Lautner, who has inspired a near cult-like following, begins filming this week on his new film. He is playing a teen who finds his own baby pictures on a missing persons website.