Why? We know damn well that Steven Seagal (or whoever) didn't really perish in a poorly-shot frenzy of snot and screaming while on a spook-hunting weekend down a disused mineshaft last summer. We know this because we saw him several months later in that new thing where he jumped a sports car through an exploding helicopter and spin-kicked a bad guy into a volcano, etc.
Basically, if viewers are unable to convince themselves that this might just have really happened, you're on to a stinker from the outset. Anyone earning more than $300 for their time is pretty much a no-go, then.
See: Pretty much every successful found footage horror out there. If any of these 'rules' could properly be considered golden, it's this one.
Don't: Well...put Steven Seagal in it. For any reason. Seriously. Please.