Chapter 82: The Imagery of the Starry Sky
Professor Flitwick continued, "Your observation is correct.
The Patronus Charm is the most direct example. It requires the caster to draw upon a deeply positive force within themselves. The quality of that force directly determines the Patronus's form and strength.
If there isn't enough corresponding light in your heart, the spell simply won't ignite."
Regulus listened quietly. The Patronus was familiar territory for him.
"In theory, any spell can be infused by the caster's inner state," Flitwick went on, deliberately borrowing Regulus's word.
"However, that infusion is usually unconscious and fluid, and very difficult to pin down.
Attempting to control it deliberately can be dangerous. Forcing your emotional state to conform to a spell may cause unexpected changes in the result.
It might turn out well. It might not. If it turns out badly enough, it can even cause damage on the level of the soul."
"But it isn't impossible," Regulus said.