The practice is there to help when your head's full of adrenaline.

That's fair, but it's only part of it. The issue of "stage fright" still exists and can't be mastered just by practicing your craft to proficiency in solitude. There are other practices, like literally going on stages (speaking, performing) and becoming used to the pressure, that would overlap.

Hopefully I'm not giving useful tips to future assassins. :P