The audience is never wrong. Which is why Irving Thalberg put great faith in preview screenings to shape the pictures he supervised during the glory days of MGM.

Hollywood’s original boy wonder executive did not invent the concept of seeking feedback on a film by screening it to an audience far removed from studio insiders, but he perfected it for his own needs. The proof remains on the screen, particularly in productions supervised by Thalberg ranging from “The Big Parade” and “Mutiny on the Bounty” to “A Night at the Opera” and “The Good Earth.”

“He wasn’t the first to do preview screenings, but he was the one to use the preview process systematically in a way that others had not,” says Mark Vieira, Hollywood historian and author of the 2010 biography “Irving Thalberg: Boy Wonder to Producer Prince.”

Thalberg and his staff traveled by private trolley car to Glendale, Pasadena, Pomona and other California cities for previews so often that the Pacific Electric trolley company laid tracks that led right into the MGM lot.

Once a screening was under way, Thalberg would move around the theater to observe the audience’s body language. More than the comments collected on the cards passed out at the end, Thalberg wanted to see how regular folks reacted — or didn’t — to key moments, lines of dialogue, actors, etc. If he saw a lot of people shifting around in their seats, he knew the picture wasn’t doing its job.

“Movies aren’t made — they’re remade,” Thalberg once said, according to “Boy Wonder.”

Then as now, preview results often illuminated plot holes, or points that needed to be made clearer, or material that needed to be cut to keep the story moving. A key scene in “Grand Hotel” in which Greta Garbo finds John Barrymore stealing her jewels was adjusted significantly because auds didn’t get the subtle way that Barrymore’s motivation was telegraphed. The 1925 John Gilbert silent “The Big Parade” became much more of a war epic after an

exhib praised the battle sequences following a preview.

Thalberg was known for keeping the sets of his movies standing long after principal photography finished in order to shoot retakes and new material as warranted from the preview results. For high-profile projects, particularly those starring his wife, Norma Shearer, Thalberg would often hold as many as three or four previews prior to release.

Thalberg’s search for preview auds repping a cross-section of the movie-going public stretched as far as the Bay Area and San Bernardino. According to “Boy Wonder,” he had a strong affinity for the Huntington Park area of L.A. near the U. of Southern California campus.

The exec, who had headed production at Universal at age 21 before he joined forces with Louis B. Mayer, believed almost any movie could be made better with prerelease audience feedback. But, sometimes, previews told Thalberg when to cut and run.

When Tod Browning’s 1932 cult classic “Freaks” previewed to a Huntington Park aud, some people ran out of the theater less than halfway through the film. Thalberg responded by cutting the movie’s running time to 63 minutes and killing all publicity for the release.