Sigourney Weaver looked every bit as big as she did in "Alien" when she walked into her favorite bakery for breakfast near her Upper West Side studio apartment. Six feet tall, she is an altogether striking woman with the skin and hair of a Gibson Girl.

But warrant officer Ripley she is not. "She's much more courageous than I am," said Weaver, a somewhat shy woman who grapples with quotidian nuisances that Ripley would have conquered without thinking. Her latest problem was fleas, left all over her apartment by three kittens of a stray cat that mysteriously entered it while she was in Japan promoting "Alien."

What, she asked plaintively, does one do about fleas in the furniture?

It is an understandable question. Sigourney Weaver never sold newspapers after school to pay for acting lessons. She is not one of 14 children. Her father ran neither a laundry nor a modest corner grocery store. There is simply no such angst in her early years.

Most Hollywood publicists, in fact, would label her childhood unacceptable. But at 29, Weaver appears to have cleared the final hurdle to success that confronts anyone with a silver-spoon heritage.

Her father, Pat Weaver, ran NBC during the 1950's and, like his daughter, was a Newsweek cover story. She survived a comfortable childhood on Manhattan's Upper East Side and was schooled at Ethel Walker's in Connecticut, then at Stanford and Yale drama school.

This summer, after five years of off-Broadway productions and some television work, all six feet of her appeared on movie screens across the country as Ripley in the sci-fi hit, "Alien." Ripley was a formidable presence - to some, a feminist's dream who eschewed coquetry and radiated competence and authority beyond her years.

Even critics who were not generally enamored of the movie praised Weaver for her portrayal of Ripley, the character who finally succeeds in killing the monster. Comparisons were made to a young Jane Fonda. There was the Newsweek cover. She had, in short, gotten off to one of the faster starts in motion pictures in recent years.

"I can't believe how different I am from Ripley," she said over eggs and toast. "But it took me a long time to like the character. She was not in touch with her feelings. It wasn't her job. I just couldn't live that way."

"The human relationships are so bleak in "Alien,"" she continued, "I told them that when they were filming it. There was one scene where Ripley comes into Dallas' [the ship captain's] room and takes off her jumpsuit and says, "I need some relief," but that ended up on the cutting room floor."

It is hard to believe after observing Ripley on screen that Weaver's love is comedy. But that's one of the elements she is looking for as she sifts through the scripts that have been pouring in recently.

"My favorite experience is live comedy," she said. "I did comedy in some off-Broadway productions after I got out of Yale, and I loved it. It was an incredible change to do "Alien.""

"I showed up at 20th Century-fox in hooker boots up to my knees," she added with a smile. "I thought that since it's science fiction, I could wear anything. I had this terrible attitude toward the movie when I went in to discuss the part. I expected some yellow blob or something."

Close. The blob turned out to be metallic black - made from pounds of Hollywood monster flesh and K-Y jelly, and worn by a 7-foot African Masai tribesman (a design student in London) who was hired for the part.

If it is difficult to imagine her tossing out one-liners, it is not at all hard to picture her in a John Cheever story about upper middle-class suburban WASPs. But in between promotion trips for "Alien," she is filming Cheever's "The Sorrows of Gin" for public television in Hartford with Ed Herman and Eileen Eckhardt.

She also played the young wife of a rich New York banker at the turn of the century in last year's PBS series "The Best of Families" in which all of her patrician bearing was displayed, if somewhat woodenly.

While she says she is not afraid of typecasting, Weaver would not be adverse to a different image. Good comedic scripts, she is learning, are rare commodities. "I've been looking for parts like "Bringing Up Baby" but I haven't found any," she said. "But my comedy will come out. Eventually, I'll hit one of them."

One has the feeling she will. Take her name, for example. She came into this world with the pleasant, if unremarkable, Christian name of Susan. By the time she arrived at Ethel Walker's, she had concluded that it was boring and started looking for a new one.

She found it in F. Scott Fitzgerald's great prep-school staple, "The Great Gatsby." Weaver saw a "Sigourney" listed in the beginning of the book as one of its West Egg characters. It sounded good in English and even better in French. She saw it again in another one of Fitzgerald's books, "This side of Paradise," and it stuck. Enter Sigourney Weaver.

While she grew up without material want, she has seen her fair share of unemployment lines and pitiful off-Broadway salaries since leaving Yale in 1974. "My father told me if I'd spent three years at law school there, I'd be making a lot more money."

But she knew, too, that implicit in his statement was a message: Do what you want to do. "I suppose that's one of the advantages of the kind of background that I have," she said. "It gave me the security not to worry about succeeding in financial terms alone. It gave me that freedom."

Weaver is on the threshold of big money now. Having recieved what she describes as "a very good salary for a first movie," she now will be looking at six figures in her next contract.

A loyal Upper West Sider, she will acquire a new apartment in the same neighborhood soon. And she will have much more financial freedom than her modest amusements require.

Weaver remains a stranger to the New York disco scene. She goes out rarely and guards her privacy with a small circle of friends who predate her success in "Alien." Her therapy is dancing and singing lessons and an occasional movie. Whether it is because of taste or shyness, she steers clear of the events that wind up in the newspaper party pages.

Money will not do much to Weaver, one suspects. She has seen it up close for years, and there simply is no novelty there. Far more important was the phone call from her father welcoming her to the club when she found herself on the cover of Newsweek last month.

Sigourney Alexandra Weaver had an appointment after breakfast with a producer to discuss the role of a prostitute in a new script. As she folded herself into a cab and disappeared into New York traffic, it was difficult to believe she was going anywhere but the Russian Tea Room.

That's why, if she takes the role, she'll probably be playing the most expensive hooker in town. Catherine Deneuve, move over. CAPTION: Picture 1, Dan O'Bannon, who dredged the story of "Alien" out of the turmoil of his life. by Joel Richardson - The Washington Post; Pictures 2 and 3, Sigourney Weaver who played warrant officer Ripley in the science-fiction horror movie. by Donal F. Holway