Craig Blankenhorn, Fox
Leonard Nimoy guest starred in the first-season finale of "Fringe."

PASADENA, Calif. — Leonard Nimoy, who made one guest appearance on "Fringe" last season in a pivotal role, will return in Season 2. And, according to the show's producers, he's welcome to appear any time.

"As much as he wants, truly," executive producer Jeff Pinkner said. "We've made an open invitation. We've already filmed one of them."

"There will be several more for sure," added executive producer J.H. Wyman.

(The second-season premiere of "Fringe" is scheduled to air Thursday, Sept. 17, at 8 p.m. on Fox/Ch. 13)

Anna Torv, who stars as Olivia Dunham on "Fringe," said it was "extremely" cool to have a sci-fi TV legend like Nimoy — most famous for his "Star Trek" role of Spock — on the set.

"Unfortunately, the set wasn't very cool when he was on there, though," she added. "It was like a heat wave in Vancouver."

"Fringe" is produced in Vancouver, British Columbia, where you might think it would be cooler than, say, Los Angeles. But that isn't always the case.

"It was 106 degrees outside," Pinkner said. "And unlike Los Angeles or New York, the sound stages in Vancouver don't have air-conditioning. It was 120-some degrees inside, and like a pro, (Nimoy) sat there all day. He never went back to his trailer, and did pages and pages and pages of dialogue and scenes with Anna."

"Well, he's from the planet Vulcan," joked Joshua Jackson who stars a Peter Bishop. "He's used to that kind of heat."

And, while Nimoy didn't perform any Vulcan mind melds, he did apparently use mind powers during his guest stint.

"Sincerely, his wife told me that he practices biofeedback," Pinkner said. "And he just sort of, like, regulates his body temperature, which is very Spock-like. ... I was dying."

"FRINGE" FANS who saw the first-season finale in May were somewhat stunned to see Olivia looking out a window in one of the World Trade Center Towers. And, yes indeed, she did cross over into an alternate universe.

A universe where William Bell (Nimoy's character) has been sort of hiding.

There has been some confusion in some quarters, however. Apparently, some viewers are under the impression that Olivia traveled back in time to before the World Trade Center was destroyed by terrorists.

"We have decided that though science acknowledges a multiverse and an infinite number of universes, we are only going to tell a story about two — here and what we are referring to internally as 'over there,' " Pinkner said. "But they are two versions of reality. It's not time travel."

That two-dimensional plotline will, not surprisingly, play a huge part in the upcoming Season 2.

"It takes place predominantly over here," Pinkner said. "But what's happening over there is impacting what's happening over here."

OK ...

The use of the Twin Towers was something the writers debated.

"We are challenged by the best and most succinct way to convey the science. ... How do you convey a parallel universe?" Pinker said.

(And, by way of justifying the plotline, he went out of his way to say that "Science has actually acknowledged or hypothesized for decades the idea that there's literally a doppelganger for each of us living in another universe where very similar, although different events are taking place.")

So, when the writers/producers mapped out a season finale in which Olivia crosses to "over there" to meet William Bell, "We tried to figure out what would be the most iconic symbol to represent this is very much like here but different."

Eventually, they landed on the World Trade Center "as the visual representation of that — such an iconic thing that tragically is no longer here. And we immediately thought, 'OK. that's the perfect (thing). That's the end of "Planet of the Apes" when you'd see the Statue of Liberty buried in the sand.' "

But they were concerned that it might still be too soon to use that image in an entertainment program.

"We struggled with whether it would be appropriate or disrespectful," Pinkner said. "And we thought that, ultimately, it's an honest way to represent that there's another version of Earth where different choices were made and, therefore, different consequences."