4 ideas
Sophie, the eldest of three sisters, resigns herself to a dull life trimming hats until the Witch of the Waste turns her into a ninety-year-old woman. She walks into the moving castle of the wizard Howl and strikes a bargain with Calcifer, the fire demon powering it: she will break his contract with Howl if he breaks her curse. Released from the expectations of youth, she becomes bossy, blunt and brave in ways her younger self never dared.
Sophie is sure that eldest children in fairy tales fail, and she lives down to that role long before any witch acts on her. Her real magic is that her words bring things to life, and she never notices it because her story about herself leaves no room for it.
Once Sophie looks old, she no longer has marriage prospects, appearances or deference to protect. Nothing is left to lose socially, so she says and does what she actually wants. Read this way, the curse works as a release: a lowered status removes the audience a person was performing for.
Howl gave his heart to Calcifer, a falling star, in exchange for Calcifer's life and power, and the two are now bound so that if one dies, the other does too. The deal keeps both alive but leaves Howl evasive, vain and unable to commit, always slipping out of obligations and relationships.