Walnut Street Theatre''s holiday production of "The Wizard of Oz" promises to be one of the most lavish musicals ever staged by this historic Philadelphia theatre, with a cast of more than 50 and special effects galore.

The Walnut brings this timeless classic to the stage Nov. 18 through Jan. 7, using the adaptation by Britain''s Royal Shakespeare Company. In addition to recreating your favorite aspects of the famous Judy Garland movie, the stage version includes material and songs written for the film, but not used in the final version. It''s he "Wizard of Oz" we''ve always loved, and more.

Exactly what''s involved in taking "The Wizard of Oz" from screen to stage? Judging by what''s been going on behind the scenes at the Walnut, it started with months and months of planning.

THE POWER OF PYROTECHNICS

The Walnut production staff has been working since last summer to obtain a stunning array of pyrotechnics, including flames, colored smoke and sparks. Flames for the Wizard, smoke for the Witch, sparks for her broomstick. Even the twister that whisks Dorothy from Kansas to Oz calls for some pyrotechnic wizardry. FLIGHTS OF FANCY

Flying is not uncommon in the Land of Oz; everybody and everything takes to the air at times. Dorothy, Toto, witches good and bad, houses, boats, chickens and trees. The production staff began making special arrangement for these effects last summer.

CASTING CALLS

The massive job of casting "The Wizard of Oz" also began last summer, with an open audition call for small terrier-like dogs to play Toto. Forty-three proud dog owners responded and brought their pets to the Walnut. Judged as the most charismatic canine was a dog whose name really IS "Toto," owned by Michael Robert Kelly. His dog "Wica" will understudy the role.

A few weeks after Toto try-outs, we held auditions for local children to play the 20 Munchkins. Casting Associate Karen Hinton coordinated auditions for 80 children, many more were interested but had to be turned away because audition appointment slots were filled so quickly.

With Toto and the Munchkins accounted for, we proceeded to cast the adults. Among them is Lisa McMillan, who was acclaimed for her supporting role as Vera in last season''s holiday production, "Mame." McMillan, who gets a chance to show her sinister side in "The Wizard of Oz," in the dual roles of Miss Gulch/Wicked Witch of the West.

SETTING PLACES

Just as Kansas and the Land of Oz were created on the MGM lot, the Walnut''s production staff is currently building those fantasy locales in our warehouse on North Front Street. We''re recreating the dusty Kansas prairie a quaint and colorful Munchkinland, a haunted forest, a shimmering Emerald City. Research on this phase of the project began as far back as July.

NEW MATERIAL

If you think you know the original film by heart, you''ll find some unfamiliar but delightful material in this stage version.

For example, you won''t see the "Jitterbug" in the film, but you''ll see it on the Walnut stage. The "Jitterbug" is a rousing song and dance number performed by Dorothy, Scarecrow, Tin Man, the Lion, a host of jitterbugs, ghosts and others. the Jitterbug was sent by the Wicked Witch to cast a spell on Dorothy, forcing her to dance until she drops. the Witch''s theory was that an exhausted Dorothy would put up little resistance to being kidnapped by her henchmen.

In the movie, you hear "Somewhere Over the Rainbow" sung only once. But in the stage version, a frightened Dorothy, imprisoned in the Wicked Witch''s castle, sings a reprise. Audience members get two chances to savor the lovely ballad that won an Academy Award and went on to become Judy Garland''s signature song.

"The Wizard of Oz" is under the direction of Charles Abbott. He is a veteran at directing the Walnut''s holiday musicals, with such hits as "Me and My Girl," "Joseph and the Amazing Technicolor Dreamcoat" and "Jesus Christ Superstar."

Ticket for this extravaganza are now on sale. Demand for tickets has been strong for the start, after the Walnut announced that "The Wizard of Oz" would be part of the 1995-96 season. Call (215) 574-3550, ext. 4, from 10 AM to 10 PM daily for tickets.

-- From Philadelphia Playbill