Diva behavior abounds on the set of CBS' Zoo. According to executive producer Jeff Pinkner, it's not uncommon for an actor to go completely off-script, throwing a scene into chaos or to have a tantrum and stubbornly refuse to participate in filming.

Of course, the actors in question are all extras - and non-human ones at that. (Though some of the animals are added into scenes using CGI, the vast majority are real.) Among the more memorable guest stars who've been on set thus far are wolves, rats, a snake, and a 600-pound bear.

"Dealing with real animals is both a joy and one of the many complications of making a show like this," Pinkner tells TVGuide.com. "The bear was supposed to grab and open a refrigerator, but the door didn't quite open and the bear instead pulled the refrigerator over - and then got scared off and ran across the stage. ... [Or] you can say 'Okay, ready, action,' and the bear has fallen asleep. And there's nothing you can do about that."

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"When there are real animals on the set, all the actors just behave differently. They're scary, and they're unpredictable," Pinkner says. "Interestingly, some of our actors have had surprising fears of animals that you wouldn't expect, and then been unafraid of animals that you would think that they would be. I won't sell anybody out. ... [But] we are all animal lovers, first and foremost. The animals have been probably treated better and with more kid gloves than the human beings on our show."

Despite Pinkner's assurances, PETA issued a statement this week condemning the show's use of wild animals, particularly lions. In response, producers told TVGuide.com in a statement: "Everyone involved with Zoo has tremendous respect for PETA and its cause. The care, safety and overall well-being of any animal on our set is a top priority of the production."

It all sounds very meta, given the fact that Zoo (based on the novel by James Patterson) chronicles a coordinated uprising among members of the animal kingdom against their human oppressors. The comparison isn't lost on Pinkner.

"Our cast has been unbelievably adventurous and patient and courageous," he says. "Every single day, acting with the animals or working with the animals has been an adventure. Because no matter what, no matter how well-trained they are, what you get is not what you expect, and that's part of one of the underlying themes of the show. At the end of the day, they're unknowable and they're unpredictable, and they're going to do what they want."

Zoo, which CBS ordered straight to series last July, stars James Wolk as Jackson Oz, a zoologist who suspects that an animal uprising might occur after witnessing strange behavioral patterns among animals in Africa. Elsewhere in the world, journalist Jamie Campbell (Kristen Connolly) and veterinarian Mitch Morgan (Billy Burke) notice similarly disturbing trends.

"It's Alfred Hitchcock's The Birds, but across the entire animal kingdom," Pinkner notes. "It's just a fantastic idea. We started developing it with CBS, instantly seeing it sort of as a contained summer thriller, to a degree outside the comfort zone of what CBS does."

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However, that doesn't mean that CBS or the producers are ready to hop on the single-season "event series" bandwagon just yet. "It was important to us, and it was important to CBS, that this be more than a one-season show,"Pinkner said. "Before we ever even started writing, we kind of figured out headlines for the first five seasons, approaching each one of them as if it were a chapter in a larger story."

The series, therefore, will quickly move beyond the bounds of Patterson's source material, much like the approach CBS' Under the Dome took with the Stephen King book on which it is based. "The first time we talked to Jim Patterson, both he and we were very much of the same mind - which is, the book is the book," Pinkner says. "Anybody who wants to read the book, there it is. This is a different approach to the same basic idea. Jim has been an incredibly benevolent godfather to the project and has stayed involved, giving us his thoughts and feedback every step of the way. ... But we're going to take the basic premise and tell a different story."

Zoo also got an unexpected, unfortunate injection of timeliness this summer. In early June, an American tourist was fatally mauled by a lion at a preserve in South Africa. And about two weeks later, hundreds of animals escaped from a zoo in Tbilisi, Georgia, after a historic flood, and several were shot by police after attacking humans.

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"For the barest of an instant, we thought, 'Oh my God, CBS' promo department is really going out of control,'" Pinkner quipped, before quickly turning serious. "I do not mean to in any way make light of it. Both events are horrible and tragic. ... In no way are we finger-wagging, trying to tell people how they have to behave better ... but we have to figure out a way to share our planet with the animals. These events are horrible and specific and have happened now, but things like that happen all the time and have for hundreds of years."

But the planet can come later. For now, Pinkner and the rest of the cast and crew are still adjusting to sharing a set with other species.

Zoo premieres Tuesday at 9/8c on CBS. Get a sneak peek at all of CBS' summer dramas below.

(Full disclosure: TVGuide.com is owned by CBS.)