As horror flicks go, this one starts slowly: Woman steals money, checks into motel, steps into shower.

Cue violins — and let the slashing begin!

It’s hard to imagine Alfred Hitchcock’s “Psycho” without Bernard Herrmann’s hair-raising score. The American Film Institute agreed, rating it fourth (behind “Star Wars,” “Gone With the Wind” and “Lawrence of Arabia”) out of all the films ever made. Hear it performed live Friday and Saturday nights, when the New York Philharmonic’s string players fiddle away under a giant screen. (Alec Baldwin, the Phil’s frequent host and radio announcer, will introduce Friday’s program concert.)

“Herrmann’s music is like going on a psychological elevator,” conductor Richard Kaufman tells The Post. “It takes you down deeper and deeper into the characters.”

He says Hitchcock initially planned to have star Janet Leigh’s shower scene unfold in near silence. Herrmann disagreed, and wrote that “screech, screech, screech” segment while the director was off in Europe. “Hitchcock came back, saw it and said, ‘Well, of course, you’re right,’ ” Kaufman says. “Then he doubled Herrmann’s salary.”

Scoring the 1960 film for strings alone may have been a money-saving move. Paramount didn’t want a black-and-white film, so Hitchcock mortgaged his home to fund its $800,000 budget. But even without brass, woodwinds and percussion, Herrmann got the unusual sounds he wanted by having his players pluck and hit their strings with the back of their bows. And it works, says Kaufman, who’s conducted “Psycho” in Chicago, Ireland and a dozen places in between: Sometimes, he hears the audience scream.

For this and the Philharmonic’s other “Art of the Score” concerts (“West Side Story,” “Close Encounters of the Third Kind”), the soundtrack is digitized and the music erased, leaving only dialogue. Kaufman relies on a sweep-hand clock and precise timing indications on his sheet music to know when to cue the music.

A violinist himself, he’s played on several soundtracks, including the one for “Jaws.” It was there, he says, that he saw a veteran musician lose her cool.

“We were rehearsing the scene where the young woman’s being attacked in the water,” says Kaufman. “I noticed the violinist turning her head to see the screen. She put her violin in her lap and covered her eyes.”

Forget the shark, Kaufman says: “That showed the power of John Williams’ score!”

“Psycho in Concert” runs Friday and Saturday at 8 p.m. at David Geffen Hall in Lincoln Center, Broadway at 65th Street; tickets, $35 and up.