What angle must the blade be? Is it good to have a high attack angle, or one that’s nearly flat?
How heavy should the apparatus holding the blade be? How far should it travel? Is keen sharpening important, or does the weight do most of the work? What kind of latch/release do you use?
Lastly, morbid extra credit, did they ever execute someone with their face up, so they could see the blade coming?
EDIT FOR CLARITY: Way more complex than an axe or a noose.
They were pretty well optimised, frequently sharpened and they had an angled blade with a decent amount of weight behind it. This means that there’s reduced surface area at the point of contact so higher penetration. Guillotines don’t miss as they’ve got a guide, unlike axes which were known for occasionally gouging the victim’s back, because executioners did miss!