WHEN “Fringe” returns to night with its first fresh episode in almost two months, fans should be on the lookout for two things: clues explaining the series’ mysterious “Pattern” and what mad-scientist Walter Bishop will do next.

Played by Australian actor John Noble, Walter has become the fan-favorite character of the new fantasy series — beating out both FBI Special Agent Olivia Dunham (Anna Torv), who was supposed to be the star the show is based on, and Peter Bishop (Joshua Jackson), Walter’s not-so-estranged son.

Noble, 60, isn’t entirely surprised that fans have come to love Walter so much.

It doesn’t hurt that Walter was sprung from a mental hospital just before the series action began.

“He’s just like everyone’s strange grandfather,” he says. “A lot of our older audience relates to him because they understand some of the issues that he thinks are important — parenthood, family, morality. And the youngsters just find him very funny.”

Walter is, after all, a guy who randomly admits that he peed in his pants (“just a squirt”) and makes LSD for fun.

“Walter’s behavior can be perceived as being idiotic or funny at times,” he says. “But it might also ring a bell of terror, like a Dr. Mengele, in some ways.

“He’ll do anything in the pursuit of science and justify it to himself.”

The chance to play an eccentric drew Noble, a long-time Australian stage actor who’s better known in the states as Denethor in “Lord of the Rings,” to his first, full-time American TV gig.

While home in Australia two years ago, “my daughter [actress Samantha Noble] said, ‘Dad, this part was made for you,’ ” he says.

The series, often compared to “The X-Files,” is about a special, top-secret unit of the Dept. of Homeland Security that investigates “fringe” sciences, like precognition and dark matter.

Walter, Agent Dunham and Peter are trying to figure out what’s behind a series of other-worldly, seemingly unrelated events– including tonight’s weirdness about a young, mute boy discovered in a chamber that hasn’t been opened in 70 years — called “The Pattern.”

“It’s becoming so rich,” says Noble. “There are less loose ends and the character stuff is being developed more. I’ve always had the feeling that because the premise [of ‘Fringe’] is so complicated, it has the potential to become a television classic.”