This kinda was inspired from some rabbits of thought that ran in different directions from the safeword thread....
The question:
Who has what degree of responsibility in a play scenario?
Expansion 
: Obviously, both parties should have equal repsonsibility beforehand in communicating with one another about preferences, etc.
But in the actual spanking session itself, when the bottom is in her most vulnerable state and many times affected by emotions, chemicals, etc (such as in subspace) -- who should have the most responsibility at this time in ensuring that everything is kosher? Should the responsibility necessarily be equal at this time?
I will give my answer after reading others.
sarah