Do not do anything at all. Tested extensively, very extensively. Always the same chance to execute an enemy (which is 100% chance by default with a few restrictions as seen below), default 25% chance to turn that kill move into a decapitation. Doesn't change the 1st or 3rd person chance of execution view. Doesn't change which kill move you end up with.

Will have to wait for a mod.

Til then beware that your distance and angle to the enemy is very important, have to be in front or behind only, too far away or if your crosshairs are not close enough to the enemy, and you will kill them without getting the kill move.

As well you cannot execute an enemy with other hostiles nearby (I don't think this rule applies to sneak executions).

You also cannot execute with a left hand attack or dual wield attacks, normal or power.

You can execute with the right hand attack (with dual wielding, 2 hand, or one hand), normal or power, standing or any directional attack. Doesn't matter, as long as all these other requirements are met.

You cannot execute OR sneak execute someone if they are leaning (on the shop counter or on a post or something), sitting, sleeping, doing any other action. Standing, walking, running or attacking only.

However if you follow these restrictions, you'll find the kill moves will occur every single time even with default game settings. Tested this all a lot...hundreds upon hundreds of executions and quickloads and here I am, scarred for life by doing this testing.