The differences between the book and the musical Wicked. Obviously spoilers for both.

Elphaba

Book: Actually allergic to water
Musical: Pretends to be allergic to water to fake her death

Book: Dies rip
Musical: Fakes her death

Book: Always covered head to toe in case of rain
Musical: Usually covered, but not always

Book: Has a son! Kinda a jerk to him but his names Liir and he's cute!
Musical: Frick dem kids

Book: A lot more evil. A LOT. Still didn't deserve to die though.
Musical: Actually totally innocent

Book: Has razor sharp teeth for some reason
Musical: Nope

Nessarose

Book: No arms
Musical: Crippled legs

Galinda

Book: Never fully reconciles with Elphaba after hurting her by giving Nessa's shoes to Dorothy
Musical: Reconciles with Elphaba quickly and neither holds any grudge

Book: Marries a man outside the main cast
Musical: Marries Fiyero

Fiyero

Book: Black! Actually the victim of racism and he and Elphaba bond over that.
Musical: Pasty Lad

Book: Has diamonds painted onto his skin
Musical: Uhh. Does not.

Book: Up to interpretation whether he does become the scarecrow or Elphaba is just delusional
Musical: Canonically becomes the scarecrow because of Elphaba

Book: Betrothed to a princess outside the main cast
Musical: Marries Glinda

The Wizard

Book: Rapes Mileena (Elphaba's mother)
Musical: Has an affair with Mileena

Book: Supposedly the first male reincarnation of a demi-goddess called Ozma
Musical: Ozma isn't mentioned in depth, probably for time reasons

Dorothy

Book: Ten years old
Musical: A young adult

Book: Had a long talk with Elphaba and came to understand killing her was wrong, Elphaba got stressed and her dress caught on fire so Dorothy attempted to put her out with water, killing her.
Musical: Believes she murdered Elphaba by tossing water on her

Tin Man/Boq

Book: Marries Galinda's friend
Musical: Marries Nessarose

CONCLUSION
THE MUSICAL IS A FIX IT FIC TO REVERSE THE ANGST