If veteran British actor Richard Griffiths had his way, all actors over 55 would be issued a 3-lb. wet salmon with which to slap the face of every young, beautiful, successful upstart.

“That’s for being so lucky, you bastard!” he would shout. “And then, hit them again, if you can.”

Youth is wasted on the young, he notes. And the star of Nicholas Hytner’s “The History Boys,” based on Alan Bennett’s Tony Award-winning play of the same name, had his hands plenty full of the young. Griffiths portrays British high school teacher Hector who’s helping a group of graduating lads get a grasp of what life has to offer them.

“Each scene is its own gem, which is what I found so appealing,” Griffiths says. “Each one opens up a different side of the character, so that what you start with at the beginning is two light years from where you finish at the end. By then, you’ve seen the good, bad and ugly, and you have to decide if maybe it was a better place when he was around.”

Griffiths worked with his stage director, Hytner, who also brought the entire cast of young actors with them, filming in a British girls’ school in North London.

“It was a poor man’s way of doing the picture, for very small resources and shooting it very tightly,” he says. The challenge was balancing the energy of his theatrical compadres against the less energetic young extras.

“You use a different level of energy on camera than on stage. The temptation is to do the same stuff that you did onstage, and it won’t work, because they’re just not appropriate on camera.”

The boys proved their own challenge. “They’re so rude and bold and dangerous. You must never threaten to dare them with anything, because they will do it. They sometimes made me nervous.”

Though the film is dialogue-heavy, Griffiths’ own favorite scene makes its point without directly addressing its subject — a technique the film uses well. After being sacked for “interfering” with the boys’ studies, Hector addresses one of the lad’s sadness through discussing Thomas Hardy’s poem, “Drummer Hodge.”

“Both of them relate to the poem because of their private sorrows. Hector detects where the boy is, and then tries to talk about it in a way that will give him courage — and it’s all done without talking about it. It’s a magical piece of writing.”

Favorite film of the past five years: “The Last Samurai.” “I think Tom Cruise is a really underrated actor. He does the right thing, and he doesn’t put it in your face.”

Actor who impressed you greatly after working together: Johnny Depp on “Sleepy Hollow.” “His staying power is phenomenal. On the 22nd take, he’s still just as fizzy as he was on the first.”

Next project: “Harry Potter and the Order of the Phoenix”