Of equal importance to the role of Dorothy Gale was that of the movie’s eponymous character, the Wizard of Oz himself. When the rights to the book were acquired in 1934, it was at that time with the intention of creating the film as a starring vehicle for vaudeville and radio star Eddie Cantor. The pop-eyed singer, actor and comedian, Isidore Itskowitz by birth, was a huge sensation, but was both too busy and not as popular by the time production actually got underway
At that point, many more names were put into the hat, and nearly each candidate had a champion among the production staff.
Mervyn LeRoy wanted to cast Ed Wynn, a star of stage, radio, and film who was equally adept at comedy (for which he was best known) and drama. LeRoy thought that Wynn, born Isaiah Edwin Leopold, would bring the required gravitas, plus his trademark zaniness to his portrayal of what Dorothy called “the Great and Terrible Humbug.”
(Years later, he was the voice of the Mad Hatter in Walt Disney’s Alice in Wonderland, and appeared in Babes in Toyland, which starred Scarecrow Ray Bolger and would-have-been Princess Ozma Annette Funicello.)
Contrariwise, Arthur Freed and Yip Harburg could think of no one but the irascible comedian W.C. Fields. His full name being William Claude Dukenfield, he had built his showbiz career on an abusive, sarcastic, and yet somehow lovable character which he played so often that the line between reality and fantasy eventually blurred. Harburg went so far as to write the Wizard’s dialogue, as he doles out the presents to Dorothy’s friends, along the lines of Fields’s trademark delivery.
Neither Wynn nor Fields were interested, despite the anticipation for the film all over the country.
A name almost nobody had considered was Frank Morgan. Known in his birthplace of New York City as Francis Phillip Wuppermann, Morgan was already a prolific character actor with literally dozens of screen credits to his name. He specialized in playing befuddled ditherers, and yet possessed a gravitas and a flair for words.
Nevertheless, it basically came down to him begging LeRoy for a screen test. They let him, and he took the words intended for W.C. Fields and, to the astonished approval of the production people, made those words his own! They knew that they had found their Wizard and that he was no humbug.
To make full use of Morgan’s talents, he was cast not only as the Wizard’s Kansas counterpart Professor Marvel, but as the Guardian of the Gates, the Cabbie, and the Soldier at the Palace, eventually playing five characters, plus the gigantic head which Dorothy and her friends saw when they were finally admitted to the Wizard’s presence.
(Technically, there were two Soldiers. In a bit which was clipped from the film, the Soldier, with his upswept mustache, announced that it was time for the changing of the Guard. So saying, Morgan marched into a little hut and emerged with his mustache turned down.)