WALT DISNEY Pictures took an unusual step this year. For the first time, it allowed one of its full-length animated films to have a public showing before it was finished - with some portions in final form, others sketched out as pencil drawings.The occasion was the New York Film Festival, the movie was "Beauty and the Beast," and reaction was very positive. Spectators seemed delighted with the chance to peek "inside" a Disney animation, seeing first-hand how a cartoon looks at different stages of the production. Just as important, they seemed delighted with the movie itself - its story, dialogue, songs, and images - quite apart from the novelty of seeing it in an unfinished state. As much as I enjoyed the preview version, I wasn't certain "Beauty and the Beast" would hold up well to a second viewing. But critics are supposed to see completed movies, so I trudged off to the film again shortly before its theatrical opening. To my great pleasure, I found the picture just as enjoyable the second time around. And the reason isn't hard to figure out. Although it's a cartoon based on a fairy tale - two forms of storytelling that are traditionally aimed at children - the Disney studio has consciously molded "Beauty and the Beast" for adults as well as youngsters. Just like "The Little Mermaid" a couple of years ago, it blends child-pleasing characters and plot twists with jokes and musical styles that clearly have older spectators in mind. The film's most brilliant episode, a lavish production number in the old Busby Berkeley tradition, is sure to please children with its spectacular sights and lively sounds; but it's just as sure to please grownups who remember the Berkeley tradition first hand, from old Hollywood musicals. It's a winning formula: something for everyone! "Beauty and the Beast" was directed by Gary Trousdale and Kirk Wise from an animation screenplay by Linda Woolverton; the songs are by Alan Menken and the late Howard Ashman, who also gave "The Little Mermaid" its engaging tunes. Robby Benson and Angela Lansbury are among the off-screen voices bringing the cartoon characters to life. Everyone has done a splendid job; only the conclusion of the picture is a bit of a letdown, pulling out too many stops to achieve an ending that's not just happy but sugary sweet. The tradition of high quality lives on at the Disney studio - and here's hoping it starts a new tradition of allowing more access to unfinished animations, which countless moviegoers would surely love to sample. STEVEN SPIELBERG shares the Disney talent for appealing to different age groups, and in recent years he has entered the world of animation. What's interesting about Mr. Spielberg is that he apparently wants to bring westerns - a genre that has bitten the dust - back into style. He's attempting this through the back door, as it were - not actually making westerns, but making western sequels to completely different films. "Back to the Future III," his first experiment along these lines, was a pleasantly nostalgic romp. Now we have "An American Tail: Fievel Goes West," which continues the original "American Tail" yarn by surrounding its animated mouse hero (still fresh from the Old Country, whence he and his family came) with cowboys, sagebrush, and showdowns. As directed by Phil Nibbelink and Simon Wells, the picture is as lively as they come, full of movement and action. It also has some amiable characters, including an over-the-hill sheriff whose voice is provided by the gifted James Stewart. Flint Dille's screenplay isn't always on target, slipping into racial insensitivity when mouse "Indians" enter the story and turn out to be stereotypical savages who do little but whoop and holler. But most of the movie is energetic and cute, with appealing voice work by Phillip Glasser, Dom Deluise, Amy Irving, Nehemiah Persoff, and Monty Python veteran John Cleese, among others. Like the more ambitious "Beauty and the Beast," it's guaranteed fun for the holiday season.