In Dark Heresy, you're summoned by your Inquisitor (or an Interrogator) and told what heresy you're to stamp out
In Rogue Trader, you hear about a map leading to untold treasures.
In Deathwatch, you're handed a mission briefing and asked what equipment you'd like.
In Black Crusade... you sit on a rock somewhere in the Screaming Vortex...
How do you start a campaign or a single adventure of Black Crusade? Like no other game of the series, BC lends itself towards a sandbox approach. Further, the setting favours pro-active PCs - they're not the ones thwarting the plan of evil, they're doing the planning themselves. And other than with Rogue Trader, the focus of the game isn't Exploration, so stumbling around in darkness trying to figure out what to do isn't that much part of the theme. So... how to start?
The Compact system is suited to framing an adventure, but to come up with a plan for a compact including secondary and tertiary objectives, the PCs need to get some information first. How do you deliver that? How do you supply the details needed to concoct a Fiendish Scheme ? Do the PCs somehow take note of a planet somewhere and then do some scouting on their own without a compact drawn up? Would the reconnaissance be its own compact? Or do you just tell the PCs all they need to know about their target? Or spare yourself the trouble and simply use the PCs' background stories for adventure ideas at the first chance you get?