The blade was dull, but Scott Glenn’s reflexes were alarmingly sharp.

“Neck — eyes — face — heart — liver,” he said, taking swipes at a nervous onlooker with a karambit, a Southeast Asian knife shaped like a claw.

Anyone who has watched Mr. Glenn play Stick in the Netflix series “Daredevil” could be forgiven for wincing. A blind mentor to the superhero Daredevil, he wields a sword with such lightning-fast intensity that heads roll with their eyes still blinking.

Mr. Glenn was in town to promote Marvel’s latest installment, “The Defenders,” which will be available on Netflix starting Friday, Aug. 18, and to take a knife-fighting lesson with Dan Anderson, who owns Anderson’s Martial Arts Academy in TriBeCa with his wife, Dasha.

Offscreen Mr. Glenn, 78, is his own brand of daredevil — a freak-of-nature, ramrod-erect warrior of ripped muscle, steely sinew and nonexistent body fat who skis, hikes, races motorcycles and open-water spear fishes in the Pacific.

Mr. Glenn first came to Anderson’s last winter with the goal of learning his comic-book character’s killer moves by way of kali, a Filipino martial art that traditionally uses knives and sticks but can turn everyday objects like umbrellas and car keys into lethal weapons.

But eventually Mr. Anderson suggested that Mr. Glenn expand his technique, and they segued into Jeet Kune Do, a hybrid form devised by Bruce Lee to stop an assailant mid-attack, and which, translated, means “the way of the intercepting fist.”

“It’s more than a martial art; for me it’s a philosophy,” Mr. Glenn said. “And that philosophy is Accept and embrace and become really good at what works for you, and reject what doesn’t work for you and get rid of it.”

After stretching, Mr. Glenn met Mr. Anderson — whom he calls Sifu Dan, meaning “master,” out of respect and affection — on the mat.

As Mr. Anderson counted from one to nine, Mr. Glenn practiced a two-knife movement called “defanging the snake” on Mr. Anderson’s protected forearm, before attempting a feigned stab or slice in a vulnerable body part like the armpit, the jugular or the groin.

When creating a routine for the actor, Mr. Anderson considers Mr. Glenn’s “body geometry,” which dictates his range of movement, as well as his age, his speed and his left-handedness. He also works in a few “high-percentage movements” that can prove debilitating or even fatal if necessary in real life.

“Martial arts builds the mind, the body, and the spirit,” Mr. Anderson said. “The mind becomes very strong just from a memory standpoint. And holy moly, he moves like a 20-year-old kid.”

Mr. Glenn began to dabble in martial arts at the age of 9 to rebuild a body left gaunt by scarlet fever. It was around that same vulnerable age that Mr. Anderson, now 42, started learning martial arts for self-defense after being relentlessly bullied as one of two Asian-American children (the other was his sister) in his small Arkansas hometown.

Mr. Anderson earned certification under Dan Inosanto, a protégé of Mr. Lee, and then founded the academy in 1996. The 6,000-square-foot, two-room gym also offers classes in muay thai, Brazilian jiu-jitsu and kettlebell kickboxing, a method developed by Dasha Anderson. There are also martial arts classes for children.

Mr. Glenn practices his knife skills each day, whether he’s at his permanent residence in Idaho; on location in Massachusetts, where he is shooting “Castle Rock,” Hulu’s coming Stephen King-J.J. Abrams anthology; or on set in New York, where, in addition to the knife routine, he’ll complete a 100-rep workout of push-ups, chin-ups and assorted flies and curls in the gym of his Battery Park building.

“At my age, I know absolutely that when I do that, the next two days I’m going to feel like I was hit by a train,” he admitted. “I’m going to wake up in the morning and feel almost nauseous and not want to work out, so I’ll do patterns with knives. I know there will be one hard workout day, two bad days and then I’ll feel phenomenal.”

It’s easy to wonder why Mr. Glenn would train so hard at an age more associated with golf and water aerobics. “Now, I could say I do it because I’m an actor and this is all I’ve got,” he continued, pointing to his body. “I’m the inventory, the display window, this is it. And if I can’t play this part, I don’t get paid.”

But the real answer, he suspects, goes back to his boyhood — his rib cage protruding, his bones crooked — when he was mortified by the way he looked and felt.

“I think it probably runs that deep, because as old as I am, there’s no real reason why I should go to the gym and beat myself up,” Mr. Glenn said.

“But this place on the other hand answers a deep appetite for me,” he added, grinning as he pulled on a cap that read “Jeet Kune Do Is Everything.”

“This is huge fun.”