ALTHOUGH HE'S permanently identified with the part of wide-eyed Will Robinson from "Lost In Space," actor Bill Mumy was already a solid professional before beginning that series at age 10. He first appeared on television at age 5, and acted in almost 100 television shows, including "Perry Mason," "Bewitched," "The Fugitive" and Alfred Hitchcock and Walt Disney's programs. "You name it, I was in it," he says.

"But it got to the point where the stuff I wanted to do and the stuff I was being offered just weren't matching up," Mumy says. "I had been an actor since I was five, and I realized they couldn't all be 'The Twilight Zone,' but I didn't want to do junk. So I decided to wait it out. Luckily I didn't have to work and could explore other interests."

Mumy says he's more interested in music than acting these days. "I started writing music when I was 10," he says. "In the pilot of 'Lost In Space,' there's a family picnic on another planet, and I'm playing guitar and singing 'Sloop John B'--an old Beach Boys song--in 1998," he laughs. He has been playing guitar and keyboards with the pop group America, and has written five songs on the group's last two albums. His latest project is an album and video with a new wave band called Barnes and Barnes.

He manages to combine his acting experience with his interest in music. He just finished his part in "Hard To Hold," the first movie starring rock singer and soap opera star Rick Springfield, which is scheduled for release in September. Mumy plays a "wisecracking keyboard player kind of guy," and says he got some experience for the part when he toured as a backup musician with actor/pop singer Sean Cassidy "at the height of his teen-idolism."

Mumy also appears in "Twilight Zone: The Movie" in the third segment, called "It's A Good Life," a remake of an original "Zone" episode. Mumy originated the role of Anthony, the angelic-looking boy with a frightening power to get anything he wants. Mumy says he enjoyed doing the movie because so many "Twilight Zone" alumni came back, and he had worked with many of the technical people on his other series. He made two other "Zone" episodes, including the classic "Long Distance Call," in which he talks to his dead grandmother on a toy telephone.

Now 29, Mumy still sounds as soft-spoken and polite as Will Robinson. "I don't know how wide-eyed I am anymore," he says. "It depends on what hour of the day it is." He lives in Laurel Canyon with his guitars and his huge comic book collection (he's been collecting since age six). He travels around the country to science-fiction conventions, where he fields questions on "Twilight Zone" and "Lost In Space," and "then they give me some money and I go spend it on more comic books," he says with a laugh.

Butch Patrick also spent his wonder years in a rather alien atmosphere. As the young monster Eddie Munster on the comedy series "The Munsters," the 8-year-old actor wandered around a haunted house with his trademark widow's peak, pointy ears, deathly pallor and a pet dragon called Spot.

Patrick's first professional acting job was in a B-movie called "Two Little Bears," with Eddie Albert, Soupy Sales and Brenda Lee (who played his sister).

Before "The Munsters," Patrick appeared in "The Real McCoys," "Mr. Ed," "General Hospital" and many commercials. When the series ended, Patrick says he kept working in television till age 20, but he let his career dwindle through neglect. To support himself, he sold cars, worked at gambling casinos and trimmed trees.

"I was just young and dumb and thought I knew everything there was to know," Patrick says. "Work wasn't high on my priority list. I had a girlfriend, and I was spending most of my time surfing. But I'm not Greta Garbo--you just don't stop an acting career. It may stop by itself . . . I screwed up, but you couldn't have told me that when I was 20."

Now nearing 30, Patrick has formed his first rock band, appropriately called "Eddie and the Monsters," and has recorded a self-penned novelty song called "Whatever Happened to Eddie?" a reworking of The Munsters theme song which made its debut on Dr. Demento's Halloween radio show. Patrick plays bass and sings lead vocals on the record, and he and partner Phil Kohn have financed and produced a video, currently receiving airplay on HBO and California's MV3 network.

Whatever happened to Eddie? Yes, I'm the kid with the pointed ears/ After all these many years/ If you dare peek through my window screen/You'd turn white at seeing so much green . . . I spent my nights just howling at the moon/Or hanging out at a crazy black lagoon.

The record (with "Little Monsters" on the flip side) will be available in August, Patrick says.

Patrick says he hopes the video sparks interest in a series pilot he has written, based on the Eddie Munster character. "It would be along the lines of 'The Monkees,' " Patrick says. "There would be me and these four musicians living in this big old house. They are really four monsters Grandpa created in his basement lab, so we'd be called 'Eddie and the Monsters.' All four of them are green, and I'm in that dead whiteface."