NEW YORK, April 26 — One overcast spring afternoon, James Spader was lurking in plain sight, standing on the stoop of the Greenwich Village town house where he lives, wearing a sport coat, a fedora and a bright purple scarf, smoking a cigarette and talking on a cell phone with the producers of his NBC series, “The Blacklist.”

“Come, come in,” Spader said with eerie alacrity. He waved his hand, a gesture aimed at this reporter, although you never know with him: He could have been signalling any pedestrian who happened to make eye contact.

Up a few flights of creaky stairs to the home of this mercurial 55-year-old actor, who has played many alluring characters over a four-decade span: the manipulative preppies of “Pretty in Pink” and “Less Than Zero”; the sexual misfits of “White Palace,” “Crash” and “Secretary”; the flat-out kooks of “Lincoln” and “The Office.” Some of them had hidden hearts of gold, and others were unrepentant predators.

Like the man who inhabits it, Spader’s inner sanctum is not overtly dangerous nor unspeakably bizarre. Scattered around the apartment were coffee-table art books and the Lego toys of his 6-year-old son, Nathanael.

Yet even at a moment of maximum popularity for Spader, when “The Blacklist” has become a hit and he is about to play the title villain in the superhero epic “Avengers: Age of Ultron,” some essential part of him remains inscrutable and comfortably weird.

It is a weirdness manifest whenever Spader speaks in perfectly formed sentences, a baritone voice, an automatic pace and an unpredictable volume.

When he discusses the narrative challenges of “The Blacklist,” on which he plays reformed criminal mastermind Raymond (Red) Reddington, Spader might as well be describing his general outlook on life.

“You really can find yourself not being able to see the forest for the trees,” he said.

“And then,” he added, his voice rising without warning or provocation, “at times not being able to see the trees for the forest.”

His voice dropped to a confessional whisper: “Really, it can be complicated.”

But his acting career could not be simpler, Spader says, because there is no strategy or calculation behind it.

“You can tell there’s no plan, can’t you?” he said with a gleeful laugh. “My trajectory has not been an unbroken line. It’s always been piecemeal.”

Referring to himself in the third person as he sometimes does, Spader added: “To a great degree, one’s holding on, just trying to stay on. There are times where you feel somewhat in control of the beast. I’m not really sure one entirely is.”

“Age of Ultron,” which Spader filmed in London on a rare hiatus from “The Blacklist,” was his first exposure to blockbuster-scale moviemaking since the 1994 science-fiction feature “Stargate.” It was not much of a break from his intense television series and required him to spend many hours wearing a cumbersome light-sensitive, motion-capture costume.

Joss Whedon, the director and writer of “Age of Ultron,” said: “He had to wear some silly clothes and a helmet that shined two lights right in your face. He said to me at one point, 'I’m constantly reacting to things emotionally that aren’t actually happening.'”

But, as Spader said of the “Ultron” experience, “What’s the point of doing this unless I do all of it?”

Colleagues who have known Spader since his first flourishing in the 1980s say that he has always projected a mixture of confidence and eccentricity, naiveté and shrewdness.

Robert Downey Jr., who appeared with Spader in “Tuff Turf” and “Less Than Zero,” recalled that before they shot a frame of film together, Spader was ready to take him under his wing.

“He’s already calling me Bobby and telling me what hotel I should stay at,” Downey said.

Slipping into an affectionate imitation of Spader’s elegant monotone, Downey said: “The Chateau is mellow. It’s like a castle. The Sportsmen’s Lodge elevator smells like urine. It’s up to you.”

Since then, Downey said that, although they may go months or years without seeing each other, Spader always talks to him like he is picking up from where their last conversation left off.

“You’ll bump into him on the street,” Downey said, “and he’ll go, 'Oh, Bobby, Bobby, you know what I forgot to tell you, when we were talking about why blues music is everything?'”

When “The Blacklist” was offered to him, Spader had been looking for another television role - ideally in cable, where the seasons are shorter - but latched onto this network pilot, which offered tantalising questions about his character and an FBI profiler played by Megan Boone.

Jon Bokenkamp, the “Blacklist” creator, said that at first, Spader “wasn’t a clear fit” for the Reddington character.

But what he brought to the role and to the show, Bokenkamp said, was “a sense of humor that wasn’t initially there.”

“It is a show that is rather dark,” Bokenkamp said, “and it would be insufferable if he couldn’t bring to it a lightness and a self-awareness that can be fun.”

That impulse, “to marry a certain irreverence with gravitas,” is what Spader said attracted him to the “Avengers” sequel, which opens May 1 and in which he plays Ultron, a malevolent artificial intelligence determined to wipe out humanity. (Or as Spader calls him, “an 8-foot robot wreaking havoc in his wake.”)

Whedon said he was “looking for somebody who could do deep, hypnotic, low-register evil that your giant robot generally requires.”

The Ultron character, Whedon said, possesses “all the logic of artificial intelligence but can’t control how his conclusions make him feel.”

This, he said, made him a perfect fit for Spader.

“James can do that it’s-coming-out-of-the-subwoofer voice, and then he can do the most hilarious hissy fit and somehow make them part of the same character,” Whedon said. “As soon as I thought of him, there was never another name.”

Downey said he took a certain delight that Spader was about to join him in “the ranks of the overexposed,” so many years after they first seemed poised for Hollywood success.

Smoking another cigarette on the rooftop deck of his brownstone, Spader delighted in the simultaneous visibility and inconspicuousness he enjoys. He has a great (if diminishing) view of the Empire State Building and lives with his partner, actress and sculptor Leslie Stefanson, near their son’s school, good restaurants and friends with no connections to the entertainment industry.

When called upon to play a madman or a murderous robot, Spader said, “I’m trying to serve my own curiosity and imagination first.”

Betraying no emotion whatsoever, he added that whatever perception his performances create, “I don’t concern myself with that very much — I just do the work.” — The New York Times