Displaying emotion may be highly illogical for Mr. Spock, but tears come easier to Leonard Nimoy. The 78-year-old actor reprises his iconic role in director J.J. Abrams’ “Star Trek” prequel and debuts four days later on the season finale of Abrams’ sci-fi series Fringe as the much-discussed Dr. William Bell.

Just ask what it was like observing the scene when a young James Kirk (Chris Pine) first meets a cranky Dr. McCoy (Karl Urban), and the memories of his days with the original Bones overwhelm Nimoy with phaser-like force. “I started crying—and it still moves me now,” he says, voice quavering as he remembers his old pal DeForest Kelley, who passed away 10 years ago this June. “I loved D-Kelley, salt of the earth. I cried a lot watching this production.”

Abrams was equally emotional when, last month, he persuaded the series-shy star to watch—and then appear in—a few episodes of Fringe, which he’d never seen. “I never in my wildest dreams thought he would do it,” says Abrams. “We didn’t think we were going to get him for ‘Star Trek,’ let alone Fringe. But then we were editing ‘Star Trek’ and the William Bell question came up, and Leonard was sitting next to us and a light went off.”

Nimoy’s eyes sparkle as he recalls Abrams’ pitch: “He said, ‘We’ve got this character who is enormously wealthy, powerful and very intelligent, who was once the partner of one of our other characters [Walter Bishop]. There’s an aura of ambiguity to him. We don’t know exactly what his intentions are, but he may be behind a very negative force.’”

May be? Lest we forget, Bell performed childhood experiments on little Olivia (Anna Torv). Nimoy was intrigued, but since starring on the original Trek (1966–69) and Mission: Impossible (1969–71), he has only on the rarest occasions returned to the small screen—including a 1983 appearance on close pal William Shatner’s T.J. Hooker. Just last year, he declined an invite to guest star on Heroes alongside new friend Zachary Quinto, who plays young Spock in the film, because “it seemed too stunty.”

His ambivalence stems from previous experiences that left him feeling both frustrated and trapped. “Over a period of time on a television series, you are at the mercy of the next producer or team of writers who come along,” he explains. “If they are creative and exciting, it’s a joy. If they are not, it’s hell. You look at the next week’s script and say, ‘Oh, my God. How can we bring this all to an end?’”

Still, Nimoy had a hard time resisting Abrams—as well as Fringe and “Star Trek” producers Robert Orci and Alex Kurtzman, all of whom had treated him so well during the “Trek” shoot. Lured instantly into Fringe’s “addictive” mythology, he sent Abrams a long list of questions about the mysterious Bell, everything from “Is the character arrogant?” to “Can I wear silk pajamas?”—a request that was vetoed.

While he had fun shooting his one scene with Torv, he so far has agreed to only two more appearances next season. Abrams declines to say how many episodes he has planned for Bell, only that “he will be recurring.”

Teleporting is still outside the realm of possibility, so Nimoy’s Fringe commitment may be further limited because the series shoots next season in Vancouver, and he’s content staying put in his Bel Air mansion, which the great-grandfather and his wife of 20 years, Susan, recently refurbished. One hundred and thirty doors and windows were replaced as part of an overhaul that transformed the tiled Mediterranean hacienda into a gallery-like home with black stone floors and white walls, which display his impressive collection of modern art.

Outside his home on this April day, a team of workmen are paving over what used to be a flagstone driveway. He chuckles when asked if the Fringe role financed the tar. “Something like that,” says Nimoy. “I’m just so pleased that I’ve had the opportunity to work in some projects that have had a positive effect on people’s lives. I’m a very lucky person.” That’s a reason to get emotional.