Where The Player's Control Ends
At the tables I've been a part of the divide has been clear. Basically, if you don't define something in your backstory it is fair game for the GM to define it for you. For example, if you didn't define how you were trained it is fair game for me to introduce someone you know from your time training. More often it was things like if you didn't define that you left no evidence doing a bad thing, it was fair game for the GM to have someone pursuing you for it.
However, by extension this also meant that the player had complete control of their character's backstory and what happened. After all, if they defined it, then it was defined that way and as long as it didn't impact another PC - or only did so with said Player's blessings - that was more or less limitless. The only thing the GM could do was not allow the character into the game until points were fixed to be "acceptable" for both the player and the game world.
Now, this is all well and good for starting things off. But what about in game? Surely once the game gets going the GM has more say. I mean, they run the world, so they also decide if an army of orcs is marching down on your family's village, right? Well, yes, however that doesn't mean it's good form.
The Stories We Want To Play
As a player you have the right to not want to play certain stories. If you don't want to play an adventurer with dead parents, that is your right. By extension, if the GM then endangers or kills your parents, that isn't a cool move. Why? Because it is making a story that you specifically don't want to play with your character.
Effectively it boils down to consent. Some things are a given. If we all agree to play D&D then I can assume t hat we're going to be doing a game with combat and a heavy western fantasy influence. I can assume that you are ok with being put into combat situations. I can assume you are ok with going into dungeons and crypts and tombs. It doesn't mean I can assume that you're cool with role playing through the loss of a family member though. I can't assume you want to role play through a lot of things, and I shouldn't be doing that.
Consent Is Important
As a GM, before you go and take things from a player's backstory and throw them into your game you should ask permission. You should do this for a number of reasons, some of which are just good people handling and some of which are vital for your game. You should ask permission to:
- Make sure your player is cool with you using their ideas
- Make sure your player is cool with that aspect of their creation coming up at the table
- Make sure you have the idea right and how the player envisioned
- Make sure it's a plot that will have impact on the character.
The first two is completely about idea ownership. A player may have put something into their backstory that they don't want others using. It's their to inform part of their character and that's it. They may also be uncomfortable with their backstory coming up. This is perfectly natural. Some people are shy, and they may not want to feel judged when their backstory comes out in all the details.
The second two are important to running a good game. If you have a different - read: wrong - take on something in a backstory it's not going to play out well. At best you get an awkward encounter. At worst you have a player wondering why you're changing who their character is by changing their backstory. The last point though is also important. Maybe killing a younger brother was a big part of the character before, but the PC is now over it. Bringing it up won't bring in tension and drama, it will just be a non-issue which boils down to wasted work. No fun there either.
Backstory Should Never Be For Cheap Impact
Finally, as a rule, you should never be using a PC's backstory for cheap or quick impact. Yeah, this guy killed your parents is a fast way to get IC motivation, but it's also lazy GMing and storytelling, and your players may not call you on it but they will feel it. If a PC's backstory is coming up it should be done in a way that is respectful to the character, but also poignant for the game.
This doesn't mean you can't threaten a PC's hometown. However, when you do, you need to make it a worthwhile endeavor beyond just saving the PC's home. There's a lot of potential tension and drama in that story, and you owe it to your players to draw it out over the course of the game. It can't be a "Wham-Bam Thank you, Sir" and over. It should matter, even if only on the small personal scale.
The same should be done with any element of a character's backstory you pull out. Make it meaningful. Use them to poke and prod the character, not just to be the next mangled body found in a fridge. Put the work in. The reward you reap is more than worth it.
And if you're not willing to put the work in. If you're not going to give the backstory the respect it deserves. If you're not going to give the PCs and their Players the respect and attention they deserve on this. Then keep your hands off their backstory. Break your own toys and creations.