LOS ANGELES » While filming a movie on the Universal Studios backlot several years ago, actor Jim Carrey decided to give guests of the studio tours an extra thrill.

He jumped out from the set of the 1960 horror classic "Psycho," dressed as the film’s demented gray-haired matriarch, waving a fake blade at the frightened tram passengers.

"The driver, the tour guide, nobody had any clue what was going on," said John Murdy, creative director at the theme park. "He just came out as the mother with a rubber knife."

Carrey’s prank ranks as one of the funniest highlights in the 50-year history of the Universal Studios Hollywood Studio Tour, a backlot attraction that has evolved from a modest moneymaking idea to the centerpiece of a theme park that draws 6 million visitors a year and employs more than 5,500 workers.

The tour is now set to undergo its biggest makeover, with an overhaul of the tram vehicles, an expansion into nighttime operations and a high-tech, multimillion-dollar finale with explosions, water, smoke and 3-D effects.

"We are putting a lot of focus on the roots of the park, on the attraction that got us here today," said Larry Kurzweil, president of Universal Studios Hollywood.

The overhaul is part of a larger $1.6 billion investment Universal Studios is making to the theme park and the surrounding property that will include the opening next year of the much-anticipated Wizarding World of Harry Potter.

The investment continues a trend for the theme park’s parent company, NBCUniversal, which reported a 19 percent increase in revenue in the three months that ended in September, thanks primarily to the popularity of the Wizarding World of Harry Potter attraction that opened last summer in Orlando, Fla.

For now the Studio Tour remains the feature attraction of the park, which has grown and thrived around the tram ride over the last 50 years. It is also the most popular ride, attracting more than 90 percent of park visitors.

The tour got its start in 1964 when Universal Studios executives noticed that food sales at the studio commissary shot up after local tour buses were allowed past the studio gates to let fans get a glimpse of the backlot movie sets and props.

That led to the creation of the pink-and-white Glamour Trams, which carried about 38,200 visitors in the first year. Passengers paid $2.50 for a 2 1/2-hour tour that included stops to see a stunt show and a movie makeup exhibition.

The backlot tours, later renamed the Universal Studios Studio Tour, have since endured fires, labor strife, at least one fatal accident and a series of expansions. The tours are now included in the $92 daily admission price.

Walt Disney Co. even tried to replicate the Universal Studios tour on Disney’s Hollywood Studios park near Orlando, Fla., starting in 1989. But the attraction did not generate an enthusiastic fan following and was closed last year.

The Universal Studios tour has undergone repeated changes over the years, some by choice and others by accident, such as the 2008 fires that destroyed a giant animatronic King Kong and other backlot sets.

The oversize ape was replaced in 2010 by a 3-D version that was designed by Peter Jackson, who directed the latest remake of "King Kong."

In its 50-year history, the tour has been marred by a handful of labor disputes and tragedy, including the death in 1986 of a park employee who was run over by the tram during a special "Halloween Horror Night" show.

It was not until 1991 that the park added its first attraction that was not connected to the tour: the E.T. Adventure, an indoor ride featuring characters from the 1982 blockbuster movie.

Over the years, other additions have been made to the tour to pay tribute to major movies filmed at the studios. The trams are confronted by a mechanical shark from the 1975 movie "Jaws" and dinosaurs from the 1993 blockbuster "Jurassic Park."

The vehicles roll past the Bates Hotel from the movie "Psycho" and airplane wreckage from the 2005 movie "War of the Worlds." It also stops in a subway station to experience the devastation from the 1974 film "Earthquake."

"It’s kind of an accidental theme park in that it started as a tour and a theme park just kinda happened," said Robert Niles, a theme park expert and editor at Theme ParkInsider.com.

He compared the tour to the chicken dinners that originally drew visitors to Knott’s Berry Farm before it became a theme park.

The tour lasts about 45 minutes and is led by guides who study from a script that is about 350 pages long.

During annual recruiting drives, hundreds of potential tour guides line up at the theme park to audition for about 60 positions. The jobs are coveted because guides who want to break into the industry get free acting lessons, access to directors and producers, and invitations to movie screenings at Universal Studios.

Former Disney President Michael Ovitz once worked as a tour guide, and so did John Badham, director of "WarGames" and "Saturday Night Fever," and actor/ singer Jack Wagner of "General Hospital" and "The Bold and the Beautiful" fame.

Construction for the latest upgrades already has begun.

By summer, many backlot sets will be illuminated to extend the tours after dark. Actors dressed as Mari­lyn Monroe, Norman Bates and Frankenstein’s monster will also make appearances on the tour.

By next year the tram vehicles will be overhauled to include more comfortable seats and ultrahigh-definition monitors to show clips of the movies filmed on the lot.

The biggest investment will be a revamped ending to the studio tour.

The tram will enter a new 50,000-square-foot structure that will house a 360-degree screen, letting passengers experience a 3-D thrill ride based on the hugely popular "The Fast and the Furious" movies. Universal Studios would not comment on the cost of the latest expansion, but the Fast and Furious attraction will rely on much of the same technology as the King Kong attraction, which was reported to cost about $100 million when it opened in 2010.

"This will be the biggest upgrade we’ve ever made," said Scott Strobl, senior vice president and head of operations at the park. "This will put a cap on our tour, something everybody will be excited about."

By Hugo Martin, Los Angeles Times