This morning at a private preview of the wonderful “Wall-E,” I was startled by a ruckus behind me that I thought was a gang of intruders. I wheeled around. But it was just a special effect from the movie, blasting from one of the theater’s rear-mounted speakers.

Surround sound is nothing new, but sometimes it can be downright scary. The first time an audio track startled me was at a Hollywood Boulevard screening of “Apocalypse Now,” the week the film opened in 1979. I had heard the movie was trippy, so I was prepared for something psychedelic, but when those helicopter blades started whooshing in Martin Sheen’s Saigon motel room, I thought the theater was going to lift off.

One of the few fears that we’re born with is the fear of loud noises, which alert us to bears or terrorists. Yet in a movie theater, we learn to shut off those fears and trust that we are in a safe environment. So how would we react if, say, an earthquake really did strike the theater in the middle of a disaster movie?

I think I know. At a screening of “Bowling for Columbine” at the Toronto Film Festival, during the dead-silent surveillance footage of the two kids machine gunning the high school, an alarm went off. It turns out the theater building was on fire. Yet a dozen presumably smart movie critics stayed in their seats for another three minutes, thinking the noise was Michael Moore’s creative license. We only came to our senses when a fireman switched on the lights.