The 20th season of PBS' "Sesame Street," beginning this November, will lead up to the birth of a baby for Maria and Luis, two of the characters who married last season.

This season will also mark the 20th anniversary of two of the show's charter cast members - singer-actor Bob McGrath and Caroll Spinney, the man inside Big Bird.They recalled their beginnings on the now historic PBS children's series at a recent news conference.

"When I first heard about it, I didn't think I wanted to be involved with a new children's show, because there was nothing that exciting in terms of children's shows at that point," said McGrath, who had been featured on the Mitch Miller show and joined "Sesame Street" in 1969. "But when I saw some of the film and animation that Jim Henson and some of the other people had done, I knew this was really something quite out of the ordinary from what had been on television before."

The producers of the show expect to add new graphics and use more films, including ones by director Jonathan Demme and photographer-artist William Wegman, this season, a likely response to the trend toward multimedia kids' shows like "Pee-wee's Playhouse."

Guest stars will include Lily Tomlin, Tracey Ullman, Susan St. James, Rhea Perlman and James Galway, among others.

One of the major episodes last season was the wedding of characters Luis and Maria. This season's finale will be the birth of their baby. the producers hope to see the baby grow up on the show. Producer-director Lisa Simon said a search is on for expectant parents who would be willing to let the show follow their child's growth.

Big Bird has become the show's biggest star, but he didn't start out that way, said Spinney.

A peripheral character in the early episodes, he was sort of "the village idiot," Spinney said.

"One day, I realized he was really a child," said Spinney. "One of the scripts said he had to go to day care or something. So I said, we really should play him as a child."

Big Bird came to embody "all the fears of children and things they have to learn - a wide-eyed view of the world," said Spinney. `He gradually learned to read and write and so he grew from 4, which I felt he was the beginning, to 61/2, which is where he stays."

Spinney carefully protects Big Bird's real identity from little children, but admits when he sees a child toting one of the big yellow dolls, "I keep wanting to say, you know, (he goes into Big Bird's high voice) `Hey, I do that!"'

Spinney had been into puppeting since he was a child. McGrath had intended to have a career entertaining adults as a singer, but when rock music began to take over the business, he decided he would have to take another path.

Both men said the rewards of working for children are great, though the task is sometimes exasperating.

McGrath does a lot of concerts and benefits as his "Sesame Street" character. Children, he said, "are a wonderful audience. First of all, they're very, very honest. I had a little girl in Vancouver some years ago. I was shaking hands after a concert. She had me lean over to her and said (he drops to a stage whisper), `You know something, Bob?' And I said, `No, what?' `Up close, you don't look too good."'

McGrath at least can appear as himself to children, though he is careful when they're around. "I don't walk on red," for instance, he said.

Spinney has to be incognito, though he does have some fun. When he heard a mother call her child by name as the girl was examining a bunch of talking Big Bird dolls in a toy department, "I bent down near one and said, `Hello, Jessica. How are you? Isn't it a nice day?"'

When children visit the set they inevitably will see the man get out of the bird suit at some point. But "most of them will continue to talk to the puppet even though you're standing there holding it," Spinney said.

Spinney also does Oscar the Grouch. On the way to a meeting with Henson to discuss the character, including what kind of voice it would have, a gruff New York cabbie barked, "Where to, Mac?" and a star was born.

"I don't know who that cabbie was, but I'd like to thank him," Spinney said.