Disclaimer : IANAD.
However, the obvious flaw in the question (and the medical dramas) is that the paddles don’t restart hearts. They’re called defibrillators, as in “device used to stop fibrillation”.
Cardiac fibrillation is the *opposite *of a flatline : when the heart beats so fast or so arythmically that it doesn’t pump blood anymore. This is due to the heart’s electrical input going haywire, which happens for a variety of reasons. The paddles’ role is to apply one big electrical surge to stop the heart for a short time in order to “reboot” it. So how it really should go is : bidibidibidibidi BAMF beep…beep…beep.
And of course, contrary to what you see on any medical show ever, you never ever shock a flatline, because stopping a stopped heart is redundant. IIRC, the only way to “cure” a full on cardiac arrest is CPR or cracking the chest open and massaging the heart directly after pumping it full of adrenaline. That is, of course, assuming the cause of the cardiac arrest has been dealt with in the first place. But as a general rule, a stopped heart is bad news 
As for what kind of voltage the paddles deliver, a quick Google search gives me “three shocks, ranging from 200 to 1700V”. And finally, as for what happens when you shock a healthy person, I would guess : the same thing that happens when said healthy person puts her fingers in a wall socket or gets tased : burns, a big and possibly fatal shock to the heart, horrible pain and involuntary muscle spasms, that kind of thing.
ETA : is it ninja day ?