If you thought Keanu Reeves seemed maybe a little bit spaced-out before . . .

As a time-traveling high school dude in the “Bill and Ted” movies, Keanu Reeves blazed a path through the great expanse of Western civilization, with detours to heaven and hell for good measure. In the “Matrix” trilogy, he was Neo, the One, the hacker turned messiah who uncovers the underlying reality of our reality. More recently, in “A Scanner Darkly,” Richard Linklater’s rotoscoped adaptation of the Philip K. Dick novel, he played a drug-addled narc who slips among identities with the help of a high-tech “scramble suit” and brain-frying hallucinogens.

Reeves now continues his career-long tour of the otherworldly by assuming one of sci-fi’s most iconic roles in the remake of “The Day the Earth Stood Still,” opening Friday. The 1951 original stood apart from many of its alien-invasion contemporaries by introducing a friendly alien, Klaatu (Michael Rennie), who arrived bearing a cautionary warning for mankind as it inched toward the precipice of mutually assured destruction.

There is a certain logic to the Keanu-as-Klaatu casting (and not just because their names are near homonyms). Reeves is practically a science-fiction canon unto himself -- his other metaphysical dabblings include the cyberpunk noir “Johnny Mnemonic,” the occult thriller “Constantine” and even the alt-reality weepie “The Lake House.” Different as these films are in size and scope, Reeves tends to play subtle variations on the same slightly haunted figure: the seeker of head trips and excellent adventures. It’s entirely possible that no other actor working today has spent as much time on screen pondering the meaning of life and death and the universe.

“Science fiction is a genre I enjoy,” Reeves said over breakfast last month in an Upper West Side restaurant not far from the patch of Central Park where Klaatu and his trusty giant robot, Gort, land their luminescent globe-like spacecraft -- a considerable upgrade from the original flying saucer (not to mention the “Bill and Ted” flying phone booth).

“I guess I’m drawn to these films because there are usually really fun ideas in there,” he added. “And I like playing these characters with a weird existential conundrum.”

In fact it’s probably more accurate to think of Reeves, who turned 44 this year, less as a science-fiction avatar than as an existential hero. He’s a natural fit for the genre partly because of what one might call his cosmic blankness, a quality much remarked on over the years by his admirers as well as his detractors. Some look at Reeves and see the most inexpressive of thespians; others appreciate the Zen stoicism, the tabula-rasa minimalism. (It’s worth noting that, in addition to his gallery of sci-fi questers, he also played the soon-to-be-enlightened Prince Siddhartha in Bernardo Bertolucci’s “Little Buddha.”)

Reeves’ portrayal of Klaatu the humanoid alien benefits from the air of placelessness that he brings to most of his roles (the new film explains Klaatu’s homo sapien guise by having him arrive on Earth encased in a layer of placenta -- the “birth” scenes echo those from “The Matrix,” with their womb-like pods). Reeves is too physically striking, even in middle age, to pass for an everyman, but he has an unusually vague aura for a Hollywood star -- mysteriously cosmopolitan both in lineage and upbringing (half English, half Chinese-Hawaiian; born in Beirut and raised in Australia and Canada) and scrupulous about keeping his private life off-limits.

A fan of the original “The Day the Earth Stood Still,” which he had seen a few times as a teenager, Reeves said he wasn’t sold on the idea of an update when Twentieth Century Fox co-chairman Tom Rothman first approached him. “I’m not a big remake guy so the question was, ‘Why?’ ” he said. His interest perked up after discussions with director Scott Derrickson (best known for the 2005 breakout horror film “The Exorcism of Emily Rose”). “Scott had a why,” Reeves said. “He had a real respect, not a reverence, but a real appreciation for the original. He thought that story of the alien coming to Earth with a warning, a perspective outside of what humans can see, was a worthwhile tale, and he’s right.”

While the 1951 movie, directed by Robert Wise, tapped into the most pointed of Cold War anxieties -- nuclear annihilation -- the new film hinges on the more generalized specter of environmental disaster. (Reeves has sound green credentials, having co-narrated, with Alanis Morissette, the climate-change primer “The Great Warming.”) Klaatu professes to be a friend of Earth, though not necessarily mankind -- and in fact, may have to destroy the latter to save the former.

“We kind of inverted it,” Reeves said, comparing his Klaatu to Rennie’s. “In the original he’s more human than human, in an odd way. He’s very gentle, thoughtful, but he carries a big stick at the end. My Klaatu is more sinister and tough, ready to execute a judgment, but he eventually breaks down and becomes more human and starts to understand what a human can do.”

An amiable but reticent interview subject, Reeves doesn’t indulge in actorly tics -- there are no ruminations about his art or his process. Unfairly pegged as a dim bulb since the “Bill and Ted” days, he has often been content to play up the persona (“I’m a meathead,” he once told an interviewer). “I’m not a science fiction aficionado,” he said, almost defensively, even though it became clear over the course of the conversation that he had read his share of Philip K. Dick, Ray Bradbury and William Gibson.

He admitted to moments of retrospection these days. “The films are now kind of photo albums for me,” he said. “As I get older, I’m having that classic ‘Oh, my God, look how young I was.’ ” But in looking back at his two-decade-plus career, it was not the marquee roles (in “The Matrix,” “Speed,” “Point Break”) that got him most animated but the uncharacteristic ones: the charming comic bit parts in such films as “Thumbsucker” and “Something’s Gotta Give,” the against-type dramatic roles in this year’s “Street Kings” and Rebecca Miller’s forthcoming “The Private Lives of Pippa Lee.”

For all the jokes and jibes about his lack of range, Reeves has been more willing -- and more able -- to stretch than most people think. And he would argue that even within his wheelhouse of science fiction, that’s what he’s been doing all along. “The interesting thing about science fiction is that it’s a Trojan horse,” he said. “There’s always a romance or an adventure or drama or something else in there. You take one look and you say it’s science fiction. But that’s often just there to tame something else.”

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