Yes, I've wondered about it quite often actually, especially because the internet is full of misinformation regarding the topic of pain-electricity-mortality (e.g. voltage vs current, how pain varies in relation to frequency, etc).
Disclaimers
1. The specifics would vary depending on which country you live in as as far as I know not all power outlets work the same, but I'll try to give you a rough overview.
2. To avoid a wall of text that no one would bother to read I've deliberately simplified A LOT of things. If something is unclear just ask and I'll be happy to reply to the best of my abilities )))
What follows happens in the span of milliseconds.
The fork is inside the outlet ---> lots of current through the fork (thousands of Amps) ---> arcing + lots of heat (tens of thousands of °C believe it or not) all of a sudden ---> not-so-fun explosion ---> you're shocked and burned.
Problems:
- Your breaker would pop tens of milliseconds after plugging it in the outlet, so you wouldn't be shocked for very long and
- Depending on how you're holding the fork, the current going through you could be very high or very small (which is why some people survived it. Hell, I plugged a WET fork in a power outlet when I was a kid and I didn't even get shocked very much. Don't do it though, very dumb idea)
Depending on how you're holding the metal object the current wouldn't even go through your heart. People have died from high currents that didn't go through their heart, but as far as I know (feel free to chime in health workers) it was from the burns that followed and their agony lasted days, and for the most part we're talking about INTERNAL BURNS... hell on earth.
It sounds like I'm downplaying the possible dangers of sticking a metal object in a power outlet, so I'll add this little disclaimer to make things extremely clear: doing this is EXTREMELY dangerous and people have definitely died from doing stuff like this.
There are a lot of reasons why electric chairs aren't forks stuck to the power outlets with a switch.
TL;DR
Don't do it. It's possible to die but it will probably be excruciating and unbelievably painful. It is indeed possible to be literally
pulverized by electricity, just not from a regular power outlet