How to use character backstories

For the record, Matt doesn't really weave backstories the same way that Matt Mercer would for Critical Role. He's said his world is already there and they players are just dropped in there.

He also said that he likes to come up with back stories based on the story the DM is creating. (In the "how to be a good player video from Aug 2018)

I reccomend looking up something called "knife theory" though it might be hard to find. The basic concept is that backstories can be distilled into 'knives' named as such because the DM use them to stab the PC's with (yeah, I didn't choose the metaphor). A knife is anything that can be thrown into a situation to ratchet up the tension and encourage player investment: every named character (living or dead), every promise made, every promise broken, every secret, every kind of discrimination (fantasy racism etc.), every unaswered qustion, every faction they have a relation too, every crime they have comitted, every crime they've been falsely accused of, every person they've killed, every person they've saved, and so on, you get the idea. Write them up as bullet points, or even hace your players do it so they get a say in what they would be cool with you throwing at them, and then you can just skim through whenever you want to add one of the rogues dead parents as a vampire, or make the sister of a person they killed come for vengence, or have lie they told come back to bite them. Having between 7 and 15 knives ensures that there is room for surprise without there being so much everyone gets cut all the time (like when a character has six phobias, two alergies, are hunted by two factions and one warewolf, and juggles four dead spouses they can remember after the incident where they lost their memory). Good luck, and remember to handle knives responsably, as in don't shit over a players character but validate their backstory.