He doesn’t know what he’s supposed to expect when he enters a vessel, but he certainly doesn’t expect this.
Moriarty’s mind whirls faster than the speed of light, sees a million details in even the most mundane of actions, and it’s overwhelming. He should be able to simply sedate his vessel’s consciousness, tuck it away into a dark part of his vessel’s brain, but Moriarty fights, clings to awareness, lingers.
He does not give his body to Castiel, but shares it with him, and Castiel feels cramped.
But not just cramped. Panic—just a faint trace of it, weak but growing stronger with every passing moment—unfolds within him as Moriarty reaches out, touches, explores, his mind still spinning. He can feel Moriarty trying to flex his fingers, trying to see if he can move his body as Castiel flows through his blood.
Moriarty, Castiel thinks, stop this at once. Consenting means you surrender complete control of your body to the angel that possesses you.
But Moriarty doesn’t stop; if anything, that was an encouragement to nudge harder, push, push, and Castiel trembles as he struggles to maintain control.
Moriarty’s mind is a blur of movement, a machine calculating answers and solutions, and Castiel fears that at some point, a door leading to a dark and secret cage may open in there—and Moriarty will figure out a way to reverse their roles, imprison Castiel and be on the other side: and Castiel will be the vessel, his Grace an unending source of power at Moriarty’s disposal.
And Moriarty, he knows now, will not use that power for good.