The Simpsons creator Matt Groening was joined at Comic-Con International by actor Yeardley Smith and executive producers Mike Anderson and Al Jean and a special guest whose surprise appearance was kept even from the participants.

Moderated by former executive producer David Mirkin, the discussion began with Groening presenting himself with the 2012 Comic-Con Icon Award, which he’d actually received during the earlier Futurama panel.

Smith, who flew in by helicopter because of another engagement, arrived just after Groening's presentation. "It was a scheduling thing,” she explained. “I had given my word that I would be here, and it's very important to me, and I love the show. I started a shoe empire in the fall of 2011, and I need to be somewhere this evening that I can't miss either. So I thought to myself what do the most successful people in the world do? They figure it out. So I did. We landed on the U.S.S. Midway. It was awesome!"

The difficulties didn’t end there, however. "The weirdest thing was we can land on the Midway, but we couldn't get into Comic-Con,” she said. “At every step it was, 'I'm sorry ma'am.' I'd say, 'But I'm Lisa Simpson' and they'd say, 'No ma'am, I'm sorry ma'am. Where are your credentials, ma'am?' I said, 'I'm going to pick them up!' And they'd say, 'No ma'am" and I'm like, 'Argh! I'm supposed to be on that panel in three minutes!' So I'm thrilled to be here.”

Jean then introduced the first act of the upcoming “Treehouse of Horror XXIII,” in which “We managed to capitalize on the incredible popularity of the discovery of the Higgs boson. I thought that would get a good response at Comic-Con. Or would it?"

In the clip, Lisa has persuaded the people of Springfield to use money they’d set aside for a new Isotopes stadium to instead fund a particle accelerator. Although the device appears nonfunctional, it does eventually produce a small black hole that begins to eat objects in the town. Lisa comes up with a plan to stop feeding the black hole to prevent it from growing. However, first the Simpsons, and then the rest of the town, begin to use the hole for waste disposal, which leads the hole to reach critical mass and consume all of Springfield – except for Maggie, who manages to escape destruction. The rest of the town is spit out into another dimension, where they find aliens have built a society out of their discarded junk, including a Microsoft Zune.

The Q&A session began following the clip, with an audience member asking whether there had even been discussions of a Simpsons Broadway show.

"I think that's the one thing we hadn't thought of yet,” Groening said. “But that's a good idea."

"We like The Book of Mormon," Jean added.

"I did have an idea to have Dan Castellaneta, the voice of Homer Simpson, read Homer's Iliad and Odyssey as audio books,” Groening said. “It's only 27 hours out of his life. I mean, come on, what's he waiting for?"

Noting that the Simpsons family never seems to change, one fan asked whether they would ever undergo a permanent transition, such as Bart or Lisa moving on to the next year.

“Well, Bart is 10 years old this year, and he has been for the last 23 seasons,” Groening replied. “Next year he's going to have a birthday, and he's going to turn 10 years old."

Smith added, "We made Lisa a vegetarian one year, and that stayed, and that was a huge change. We had Paul McCartney and Linda McCartney on the episode at that time. And Lisa became a vegetarian because of them. And they said, as I understand, that the only way that they would do the show is if they kept Lisa a vegetarian. So they have stayed true to their word."

Asked whether it’s true that Matt Groening’s initials are on the side of Homer's head, the cartoonist replied, "Well, let's find out. Let's get a drawing easel out here. Good question." He then proceeded to draw Homer's head, and it turns out that there are indeed an M and a G on Homer’s head.

Groening was then asked whether he can perform any of the Simpsons’ voices well. "No,” he said. “I can do Maggie's pacifier, and I originally did Maggie's pacifier on The Tracey Ullman Show. That was me. I also did Barney's belch, and Homer's belch, but I'd need a Duff to demonstrate that.”

Asked whether they had considering making a short film based on McBain, the action hero played Rainier Wolfcastle, Jean replied, "We have discussed doing other shorts. Actually, the most common guess would be Itchy and Scratchy. So by vote of Comic-Con, Itchy and Scratchy, or McBain?"

Groening replied, "Itchy and Scratchy it is!" although the crowd clearly wanted McBain. "We love to annoy the audience," Mirkin said.

Asked in which Springfield The Simpsons takes place, Groening replied, "I've answered that question a few times, and I always give a different answer. The original inspiration for Springfield was the Father Knows Best television show, and that show took place in Springfield. So whichever Springfield was on the Father Knows Best show is where the Simpsons live as well. Springfield and Lincoln are the two most common names for towns."

Groening was asked if he enjoyed the Adventure Time display at the museum across the street, and whether there are any other current shows he likes. “Yes, I am a huge fan of Adventure Time,” he said. “In fact, Pendleton Ward, the creator of the show, when he was a young sprite, his mother brought him to my house in Los Angeles. She said, 'My son wants to grow up and run his own animated show. What's your advice for him?' And whatever advice I gave him apparently worked, because Adventure Time is a great show.”

Mirkin then introduced a surprise guest, unknown even to Groening. Carrie Fisher, carrying a blow-up Duff Beer can, stepped on stage. "She was my favorite guest star,” he said. “We've done 500 episodes, and we're trying to remember which episode it was she was on.”

"She wasn't on the show," Jean said.

"It had something to do with beer, right?” Fisher said. “When Homer went to AA?"

"No, she wasn't on the show," Jean said adamantly.

"Well, she'll have to find another panel at Comic-Con then," Mirkin said.

"That's so weird, because I'm like your best friend, right?" Fisher said to Mirkin. "I should be on the show,” she added, eliciting applause.

The panel concluded with what Groening described as a special 2D look at “The Longest Daycare,” the 3D silent short attached to Ice Age: Continental Drift.

The Simpsons returns Sept. 30 at 8 p.m. ET/PT on Fox.