It would have been easy to get the wrong idea on the set of Night at the Museum . There was star Ben Stiller being chased all over the set by director Shawn Levy, who looked like he wanted to impale the actor on a giant tusk.

Not to worry – it was all part of the process. The script of the family-oriented action-comedy called for Stiller to flee from a dinosaur. The computer-generated creature would be added later, but Stiller needed something to act off of during the filming, so Levy volunteered.

“Shawn actually made a very good dinosaur,” Stiller says in what appears to be a compliment.

In the movie, Stiller plays a night guard at the American Museum of Natural History in New York. He soon discovers that once the doors are locked, the displays come to life. The pursuing dinosaur was one of hundreds of special effects that were added in post-production.

“I had never done anything like this,” Stiller says of the FX-driven movie. “The hardest thing was not having anything to react to [during filming]. Shawn was good; he gave me a lot of help. But it was still weird.”

“Not having the character in front of you made it very difficult,” he says. “It was so much easier when you had a real human being in the shot.”

That's one reason – of many – why he was happy when Levy (who also directed this year's remake of The Pink Panther ) announced that Robin Williams would play the come-to-life version of a Teddy Roosevelt statue and that Dick Van Dyke and Mickey Rooney had signed on to play the guards on the day shift.

“Those guys are icons,” Stiller says. “Robin was everybody's first choice. Mickey made a ton of movies, and I grew up watching Dick Van Dyke. He's got to be the coolest guy ever. He's 80 years old, and he's still got the moves.”

(If you don't believe Stiller, stay for the closing credits, which include an outtake in which Van Dyke launches an impromptu soft-shoe routine when he thinks the scene is over.)

There's one other major character in the movie that's real: a monkey that has a running battle of wits with the bumbling security guard.

“The monkey was very good,” Stiller says. “I think he'd been in more movies than Mickey Rooney. He'd do his thing, and the trainer would give him a treat. He performed very well when he knew that he was going to get a treat ­ which, come to think of it, works for actors, too.”

Stiller came to trust his furry co-star so much that he even agreed to a scene in which the monkey bites the guard's nose.

“That's really my nose in his mouth,” he says.

The only complaint he had about the monkey was that the animal got a stand-in for a scene that he had to do for real. The monkey and the guard get into a tiff in which they start slapping each other. The monkey slapped Stiller numerous times in each take, but Stiller's retaliatory slaps were filmed with a computer-generated monkey.

Because adding special effects takes so long, Stiller went on to other movies, including gigs in Tenacious D in The Pick of Destiny and School for Scoundrels . He's currently in Mexico filming the Farrelly brothers' romantic comedy The Heartbreak Kid .

As a result, “I haven't even seen the finished movie yet,” he says of Night at the Museum . “Shawn showed me some stuff as they were working on it, but it was kind of strange. There were signs where the special effects were going to be added. They would say things like ‘moose here' or ‘woolly mammoth crosses.' You had to use your imagination.”

He did get a chance to see a couple of the completed dinosaur episodes, however, and they made being chased around by the director worth the hassle.

“I was really excited about the dinosaur,” he says. “It was fun to finally see it.”

© 2006, Star Tribune (Minneapolis)

Distributed by McClatchy-Tribune Information Services.

Night at the Museum releases in theaters Dec. 22.