No movie is more mythic - or the source of more muddled information - than “The Wizard of Oz.”

Opening in theaters today is “Oz the Great and Powerful,” based on the writings of L. Frank Baum and constituting an unofficial prequel to the beloved Judy Garland film.

The “Wizard of Oz” is the subject of a chapter in my new book “Hollywood Myths.” Following are some fun facts I learned about that 1939 classic.

It’s not true that a Munchkin committed suicide on camera. It is true that makeup malfunctions hospitalized at least two of the cast members. And it might be true that Pink Floyd’s “The Dark Side of the Moon” album is synced to the movie. (Check Youtube for clues.)

The 1939 film was based on the 1900 novel “The Wonderful Wizard of Oz” by L. Frank Baum. Baum was a playwright and newspaper publisher who spent much of his adult life in the Midwest. (For his newspaper in Aberdeen, S.D., he wrote editorials decrying the treatment of Indians yet urging that they be “exterminated” to put them out of their misery).

When he and illustrator W.W. Denslow created the first of more than a dozen “Oz” books, Baum was living in Chicago, where he published a magazine about department-store window displays. He would later say that the name “Oz” was derived from the letters on an alphabetized file cabinet in his office: O-Z.

“The Wonderful Wizard of Oz,” in which a girl from Kansas is blown by a cyclone to a magical land where she finds some silver slippers, was a huge hit. Baum and Denslow collaborated on a stage musical, which toured the country for almost a decade, and sold the book to Hollywood, which made a silent version in 1910.

In 1938, MGM planned a sound-and-color version of the beloved book, as an answer to Walt Disney’s successful “Snow White and the Seven Dwarfs.” But the casting of “The Wizard of Oz” proved problematic. Shirley Temple, whose film career was peaking at the ripe old age of 10, was offered the role of Dorothy, but Fox studio chief Darryl F. Zanuck did not want to lend her to a rival company. So the iconic role was assigned to MGM’s musical ingenue Judy Garland.

Garland, whom studio chief Louis B. Mayer ridiculed as his “little hunchback,” was a born trouper from a family of vaudevillians. She was 16 when she filmed “The Wizard of Oz,” yet the studio supplied her with amphetamines to meet the grueling production schedule. (She would remain addicted to pills for their rest of her abbreviated life.)

W.C. Fields was the first choice to play the title character, but MGM balked at his asking price of $100,000 and gave the role instead to contract player Frank Morgan. Accomplished dancer Buddy Edsen (the future star of “The Beverly Hillbillies” on TV) was originally cast as the Scarecrow, but he was asked to swap roles by Ray Bolger, who’d been cast as the Tin Man. After Ebsen agreed, he developed an allergic reaction to the aluminum powder in the Tin Man makeup. He was replaced by Jack Haley, for whom a new makeup was developed.

Gale Sondergaard did makeup and wardrobe tests as the Wicked Witch of the West; but when the role was changed from a slinky temptress to an old hag, Sondergaard bowed out, and the role went to character actress Margaret Hamilton.

Hamilton was an on-set casualty when her green makeup caught on fire during the scene when the witch leaves Munchkinland in a puff of smoke. She needed six weeks to recuperate from burns to her face and hand.

As with “It’s a Wonderful Life,” television turned “The Wizard of Oz” into a beloved classic. It first aired, on CBS, in 1956. Although most people did not have color televisions to fully appreciate the Oz sequences, the broadcast was watched by an estimated 45 million viewers. The annual broadcast was a Christmas-season tradition until 1967, then it rotated through various holidays and networks for another 30 years.

Between theaters, television and home video, “The Wizard of Oz” is believed to be the most watched film in history.