Existing doesn't come FREE for any creature, all are obliged to do some INANE task to prolong their existence and Bill is no different. Once every thousand years he MUST consume the life-force of some living thing, the catch being that the creature in question has to AGREE in order for him to take what he needs. More often than not it's a TEDIOUS process, he must offer something they want and in return they are to give him their life force on the day of their death. WILLINGLY. That means no dirty tricks, no killing them himself, he must play NICE.
The rift in Gravity Falls makes the Humans there EASY prey, he can slip into their world and make contact with someone who he thinks will give him what he needs. It takes time for him to choose, WATCHING and lurking in the dreams of townsfolk until he finds one that doesn't seem too COWARDLY to fulfill their end of the bargain.
Even once he's found someone, the approach must be CAUTIOUS. Firm enough not to seem like a mere dream but not so much that it FRIGHTENS. In the end, he settles for making his presence known in a flash of light, almost DIVINE in it's approach as his hands hold glowing embers, the dream around them fading away.
❝Wendy Corduroy. -- I'm here to have an IMPORTANT
conversation with you, you might want to sit down, kid. --❞
It only takes the CLICK of his fingers to summon up a chair behind her and a cup of coffee floating beside it, his best attempt at seeming WELCOMING.
❝-- The name’s BILL CIPHER, your guardian to be.
That is, if you say YES.❞