On The WB's "Smallville" (tonight at 8) — 6-footer Michael Rosenbaum plays cool and stylish Lex Luthor, a tycoon destined to be the nemesis of young Clark Kent (Tom Welling) when he eventually becomes Superman.

In real life, Rosenbaum hasn't always been cool — or tall.

"I was 5-2 when I graduated high school," Rosenbaum says. "I grew 9 or 10 inches in college. That's probably why I have back problems, right? I'm an unnatural human being with an unnatural childhood. We were Jews from New York who moved to Indiana."

"I never went to a high school dance," he says. "I never went to prom. I baby-sat my brother on Friday and Saturday nights, didn't go out. My parents were out gallivanting, and I'd be sitting there.

"I'd be watching Saturday Night Live, and they'd come home, and I'd do the show for them. I'd do every character, Church Lady, everything. So not only would they party every night, they'd come home and have a show for themselves. And they wouldn't even pay me."

But rather than brooding over his childhood misfortunes, Rosenbaum decided to put them down on paper, with a little inspiration from actress and writer Carrie Fisher.

"I started going to her parties and meeting her friends," he recalls. "She's a very eccentric woman. Anyway, I read Postcards From the Edge, and I thought to myself, 'Hey, her life was pretty screwed up, and she profited from it.'

"I thought, 'I remember my childhood like it was a movie.' People don't understand when I tell them these stories. I've always been a bit of a storyteller, I guess."

So Rosenbaum started to write short autobiographical sketches and showed them to Fisher. "She goes, 'It's pretty funny, Michael. You might want to do something with that.' "

During a flight, Rosenbaum showed the sketches to Smallville executive producer Greg Beeman.

"He's bellowing with laughter," he says. "I thought we were going to be kicked off the plane. He's like, 'This is a TV show.' Next thing we know, we get together, and in five sessions ... we have a script."

Last fall, Fox gave a script commitment to Welcome to Paradise, Rosenbaum's fictionalized account of his Indiana childhood. While it's not on track for fall, it remains in development at Fox Television Studios. But Rosenbaum has already benefited from the experience.

"Greg goes, 'Rosenbaum's too cheap for therapy. This is his therapy.' I'm blessed. I really am. I love writing. I try to do everything. That's my problem. I do a lot of things, but not always great. I like to play guitar. I like to write. I like to act. I want to direct. I'm doing documentaries now. I'm editing them."

Welling is directing a Smallville episode this season, and Rosenbaum is planning to do the same (especially since the show seems a lock to make the transition to the new CW Network).

"They said I could do one next year," he says. "I figure, 'Why not?' "

It looks as if Clark's on-again/off-again squeeze, college coed Lana (Kristin Kreuk), might now be drawn to older-man Lex. In a December episode called "Lexmas," Lex dreamed that he and Lana were married and had children.

"That's heating up," Rosenbaum says, "and who knows what's going to happen? By the way, it was a dream, and sometimes you have a dream, and certain people come into your dream. I said to one of my closest friends, 'I had a dream that we (made love) last night.' She's like, 'Yuck.' I'm like, 'Hey, it was a dream. I didn't want to dream it.' "