[Quick Summary: A tornado sends Dorothy to Oz, where she befriends the Scarecrow, Tin Man & Lion. She must bring the Wicked Witch of the West's broomstick to the Great Oz in order to get home.]
Usually I see:
1) subplots that are disconnected from the main plot.
2) subplots w/ minor characters that try to stand out too much, i.e., They think this is their movie.
Wizard of Oz stands out b/c all the subplots support the main character's goal, i.e., Dorothy's journey home.
This is especially clear in Act 3 when Dorothy isn't around:
ex. The Scarecrow uses his brain to get into the castle to rescue Dorothy.
ex. The Tin Man cares so much that he breaks down her jail door.
ex. The Lion faces his fear and uses courage to rally the troops when he'd rather back down.
Why does this work?
Because the supporting cast are all invested in the main character.
Because they know this is Dorothy's movie, thus they are working toward Dorothy's goals. Yes, they get their goals met too, but only b/c they're in service to the main character's goals.
WHAT I'VE LEARNED: Subplots should echo the theme back to the main character.
Here's Dorothy realizes "there's no place like home" b/c each of the subplots remind her how much she took the Kansas farmhands for granted.
Wizard of Oz (1939)
by Noel Langley, Florence Ryerson, and Edgar Allen Woolf