I've heard people say that a defibrillator being used on someone whose heart has a normal beat, like if they have a blocked throat and can't breathe, but still have a regular pulse. However, I have also heard that defibrillators only stop and restart the heart. Wouldn't people then suffer the same risk from defibrillators if their heart is functioning normally as if it were stopped?
Depending on the type of the defibrillator. The Automated external defibrillator (AED) units that are commonly available in airports, railway stations, government agencies, shopping centers and other places with lots of people, these are not dangerous to apply. The principle is that these "to-be-used-by-non-professionals" devices are "clever": they detect the conditions of the victim, and apply the shock only when fibrillation is happening, and in coordination with the specific detected fibrillation waveform. This means that they cannot operate on dead bodies, or on people with normal cardiac activity. For more information on these see: http://en.wikipedia.org/wiki/Automated_external_defibrillator
This is not the case of the medical defibrillators that do not detect the victim's conditions and apply the shock as the medic orders. In this case, the assumption is that the medic is watching the electrocardiogram continuously and selecting the timing and parameters of intervention in a similar (but more flexible) way to what an AED unit does automatically.
I am no scientist, but have worked as an EMT for a year.
Using a manual defibrillator on a person with a healthy heart is very dangerous. Aside from burns and other effects of an electrical shock, it is very likely to stop the heartbeat or cause ventricular fibrillation.
In general, a defibrillator is not used to start a heart but to get it out of ventricular fibrillation by either stopping it or bringing it back to a regular pulse.