Player Core
Chapter 8: Playing The Game
Movement
Tactical Movement
Moving Through a Creature's Space
Prone and Incapacitated Creatures
Source Player Core pg. 414
You can share a space with a prone creature if that creature is willing, unconscious, or dead and if it's your size or smaller. The GM might allow you to climb atop the corpse or unconscious body of a larger creature in some situations. A prone creature can't stand up while someone else occupies its space, but it can Crawl to a space where it's able to stand, or it can attempt to Shove the other creature out of the way.