So Fiyero in the musical does in fact end up being The Scarecrow. But what led to Elphie thinking he was The Scarecrow in the book? Maybe I'm missing something, but the line of reasoning seemed to come out of nowhere. I've yet to read the sequels.
Why did Scarecrow help Dorothy go see the Wizard?
(Book) Why did Elphie think Fiyero was The Scarecrow?
Why would Fiyero help Dorothy defeat the Wicked Witch as the Scarecrow?
Wicked Fiyero Scarecrow confusion
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Fiyero just left the Wizard's guard to be with Elphaba, which is the entire reason he became the Scarecrow (because the other guards were going to kill him probably). He's on Elphaba's side and knows that the things people say about her are wrong.
So, if things go as they went in The Wizard of Oz, why did he go with Dorothy and help her to meet the Wizard and potentially go along with the witch hunting? (granted, he's not on stage for March of the Witch Hunters, but neither is Dorothy, and she's the reason its happening)