Crystal, the simian star of “Night at the Museum: Secret of the Tomb,” the third installment of the Smithsonian-set comedy that bows Dec. 19, isn’t like most primates, says animal trainer Thomas Gunderson.

“She’s not the classic, typical monkey,” says Gunderson, who coaches animals to perform in the film and television industry (credits include “American Pie” and “The Hangover III”) and has been the caretaker and trainer of Crystal, a capuchin monkey with an extensive theatrical resume, since she was born about 18 years ago.

“Most monkeys tend to be rebellious or overzealous — they don’t like people telling them what to do,” Gunderson says. “But Crystal doesn’t like to just goof around. She’s very smart about figuring out things. She likes to do activities that exercise not just her body but her brain.”

Gunderson, who is employed by Birds and Animals Unlimited, has worked with Crystal on more than 90 projects, including former NBC sitcom “Animal Practice” and the first two “Night at the Museum” movies. The trainer lives on a three-acre property 30 minutes east of Los Angeles with a menagerie of furry friends, including Crystal, whom he considers a client in a business relationship that has him playing the role of Hollywood agent.

“A production company will be looking for this or that type of animal, and they’ll get in touch,” says Gunderson, who’s certified in exotic animal training, and advocates extensively for the safety of animals on TV and film sets. “Based on your motivation to train that animal and get them ready to do studio work (is what decides) whether or not you’re going to get the job.”

For Gunderson, though, Crystal is more than just a client — she’s a member of the family. “Most of the time she sleeps in bed by my leg with me and my wife,” he says. “She really likes to snuggle up with us.”