The studio did not want to let James Franco and Seth Rogen play themselves in This is the EndThis is the End is a comedy in which Seth Rogen, James Franco and a host of celebrities play themselves, facing the end of the world. Does that sound like a weird concept for a film to you? It certainly sounded weird to studio execs, who fought hard against letting a host of famous people lampoon themselves — and die horribly — in Rogen's film. That's just one of the interesting facts we learned at the movie's Wondercon panel. Spoilers ahead...

"The idea to let us play ourselves was a really hard thing," said Rogen at the panel. The film-makers had expected the studio to go nuts for it, added his co-writer Evan Goldberg. But "it really rubbed everybody the wrong way in the studio world," said Rogen. So they decided to lie, and pretend they were going to film the movie in such a way that they could always "edit around" the thing of the actors playing themselves. Thank goodness it never came to that, and test audiences actually like the result.

This apocalyptic movie was a tough sell in general, and Rogen and Goldberg had to do a ton of concept art to convince the studio to bankroll the large budget for the film, where a lot of the action takes place at James Franco's house but eventually does visit some other apocalyptic scenes around L.A. "It's a pretty big movie," said Rogen. Having so many awesome stars willing to be in the movie definitely helped though.

The self-mockery was pretty intense at the This is the End panel generally — Rogen came out dressed as Wolverine, in an old-school yellow 1970s costume, pretending he thought all the film's stars were going to be dressed as the X-Men. "I don't know how Hugh Jackman does this," Rogen said, complaining about the big yellow pointy-eared mask. Goldberg, meanwhile, came out in an S&M hood and bondage harness, because he supposedly thought they were at an adult convention.

We saw a brand new trailer, in which the biggest showcase was Michael Cera, playing a coked-up insane version of himself — he slaps Rihanna's ass and she smacks him so hard he falls over. (This is before the apocalypse, when it's just a party at Franco's house.) Rogen and Goldberg said Cera really got into playing the cokehead version of himself, and he warned Rihanna "I'm really going slap your ass." To which she responded, "I'm really going to smack you." They filmed that sequence five times, and the last time Rihanna got Cera in the ear. After that, Cera wasn't up for spanking her any more.

We also saw a clip where the apocalypse happens and everybody runs outside to see what's up — cracks and holes in the ground are opening up, and everybody's falling in and dying. Aziz Ansari is holding on to the edge of the bottomless pit, and nobody will pull him out — Craig Robinson tells Ansari it's too late for him. In the midst of this mass death, Michael Cera throws a hissy fit because he can't find his cellphone and he thinks someone stole it. Everybody freezes in the middle of running for their lives as Cera yells at them about his missing phone. Then a lamppost falls over and impales Cera through the chest, pulling him up into the air. As he's dying of his huge wound and electrocution, Cera finds his missing phone, in his pocket.

Rogen said the wacky celebrity deaths are definitely "in the double digits" in this film. Almost everybody they asked to play themselves in the film was up for it. It was a "if you build it they will come" situation, said Craig Robinson, who costars in the film. "We killed most of our favorite stars," said Rogen.

Most people got really into the concept — but a few stars didn't want to go too far in portraying themselves as assholes. And it sounds as though the on-set improve got a little brutal at times, with people taunting each other about their bad decisions and career setbacks. Feelings got hurt a few times, said Rogen. And Danny McBride joked that he spent some time reading message boards to gather ammunition before he got to the set.

We also saw a clip called "The Exorcism of Jonah Hill." The star of Superbad and The Watch is tied to a bed making demonic "rraahh" noises, with white makeup and weird contact lenses on. The rest of the film's main cast nervously straggles in, led by Jay Baruchel holding a crucifix made of kitchen implements. Hill says that Jonah isn't here any more. Jay Baruchel keeps saying "the power of Christ compels you," to which Jonah keeps saying "Does it? Oh yeah? Does it?" Until Jonah is like, "Honestly, it's not that compelling." Everybody realizes Jay Baruchel is just quoting The Exorcist, which he insists is like an instruction manual. At last, it starts to work and the bed is shaking up and down and Jonah Hill is getting upset — but the others decide Jay is just pissing the demon off, and pull him off the edge of the bed, where he's perching.

During the Q&A, someone asked about the elephant in the room — the other big apocalyptic comedy coming out this year: The World's End, starring Simon Pegg and Nick Frost. It's no big deal, Rogen insisted. "We're friends with those guys... we talked a little bit. We think that there's no overlapping, other than that they're end-of-the-world comedies. There's been communication."