Ben is correct on drilling. IMO, it should be done in 3 phases:

1. rote technique - 100% success for the person to learn the movement
2. intensity and speed - still 100% success but done at a speed and intensity closer to sparring. This will help better integrate the movement into someone's game
3. positional drilling - staying in the context of whatever the lesson is but now against a resisting opponent. This allows you to see how fully integrated a movement is and how well you can incorporate it into your game

once those things are done, then you can spar

I got some of this from Andre Galvao who is notorious for how he drills:

https://www.youtube.com/watch?v=PSYeB5DcUgE

Rome did not create a great empire by having meetings...They did it by killing those who opposed them...unknown