NEW YORK – Al Pacino, energized by a conversation that has inevitably turned to the intricacies of acting, is snapping his fingers.

“When you get me on the acting trail, I get on that train,” he says, punctuating what he calls an improvised “thesis on time” with staccato snaps.

The 72-year-old may be gray-haired and a little worn, but he remains, like a dancer, always on his toes, and still enamored of the “crazy, crazy, crazy thing” that is acting: “You’re always looking for what’s going to feed you, what’s going to feed the spirit and get you going.”

And Pacino is still getting going. Yet the subject of time – how much is needed to find a character (years in some cases, he says) and how it dictates the parts he chooses now – played a large role in a recent interview with the actor at the Waldorf Astoria in New York.

“Sometimes I’m tempted to say, ‘Why am I doing this? Why am I still doing this?’ ” he says. “Then, after I don’t do it for a while, I say: ‘Oh, now I know why I still do it.’ If I suddenly didn’t want to do it anymore, that’d be fine, too. I’d probably be an usher again in a playhouse.”

If Pacino is feeling reminiscent of his early days as a Bronx-born aspiring thespian knocking around in 1960s downtown New York theaters and cafés, it’s partly because his recent work reflects on his beginnings.

Not many know that Pacino started out as a comedian. He jokes that though he did a movie with Robin Williams (“Insomnia”), “he didn’t know that I really wanted to be him.”

Pacino, funny guy, has certainly been glimpsed before. But after a career better known for gangsters, crooks and Shakespearean villains, Pacino has lately been exercising his comedy chops.

After finishing a revival run on Broadway of “Glengarry Glen Ross” in which he played up the laughs as the desperate, over-the-hill salesman Shelley, Pacino stars in the crime comedy “Stand Up Guys,” which Lionsgate releases today.

In it, he plays a former gangster, Val, released from prison after 28 years and taken around town to celebrate by his old friend, Doc (Christopher Walken), who does it remorsefully knowing that their boss wants Val killed by sunup. Their pal Richard (Alan Arkin) joins in the romp.

As he showed in “Scent of a Woman,” Al Pacino is good company for a last hurrah. Part of his enduring appeal, after all, is his pulsating zest for life. Whether firing a machine gun at the hip (“Scarface”), pursuing a story (“The Insider”) or whipping a crowd into an “Attica”-chanting protest (“Dog Day Afternoon”), Pacino is the great agitator of American movies. Critics will make claims of overacting, but no one ever slept through an Al Pacino performance.

“Some actors aren’t connected and they don’t invest,” says “Stand Up Guys” director Fisher Stevens, a veteran New York actor and documentary producer. “Al is committed to everything he does, even if it’s just playing poker. He does everything that way.”

Pacino and Walken hadn’t worked together before (except for separate scenes in – get ready for it – the Ben Affleck, Jennifer Lopez film “Gigli”), but they’ve been friends for decades, going back to the Actors Studio, where the long-involved Pacino is currently co-president. Reading through the parts, the two decided to switch roles in “Stand Up Guys.”

His fondness for broad comedy, though, helps explain the most inscrutable credit in Pacino’s filmography: the 2011 Adam Sandler film “Jack and Jill,” in which he, among other things, rapped a pseudo Dunkin’ Donuts ad as “Dunkaccino.”

It’s ironic that the greatest accomplishment of an actor so well known for his bigness (despite his 5-foot-7 inch height) was a performance of utter control: Michael Corleone. The strain of that titanic performance – the maturation of an armchair despot through the “Godfather” films – left a mark on Pacino, who though nearly 32 at the time, had only two previous movies under his belt.

“That character was so consuming,” Pacino says. “Part of the reason why was because of its restraint, because of what is demanded of it in that style. The innards of that character, what his psyche was going through. To portray that probably affected me in some way.”

Since then, the knock on Pacino has always been that he sometimes chews scenery, or rather, swallows it whole. That’s somewhat unfair, says Stevens, who notes that Pacino tries many degrees of a character, leaving it to the director to calibrate.

But if Pacino sometimes veers into cartoon, it makes him all the more suited to comedy. In conversation, he’s every bit as lively, erratic and funny as you’d expect. “I’m throwing images at you!” he bursts between reflections. He grins mischievously when he brags that he got Stevens to open up his collar. And when the question of whether he’ll take up that Shakespearean mountain that signifies the autumn of an actor’s career, he says, yes, perhaps in a movie, but not onstage.

“King Lear? Why don’t you ask me if I’m going to climb the Empire State Building with a wire?” Pacino exclaims. “King Lear? What have I got to do with King Lear? Isn’t that for other kind of people? It’s somebody else playing it. It’s George C. Scott or Ian McKellen. I don’t do that. I’m in ‘Stand Up Guys.’ ”