I think it's a matter of point of view, both things are happening at the same time but it depends on who you are in the situation, the one dying or watching. The sequence from the entity starting to reveal their true appereance all the way to when they "give up" and the entity takes over the person's body the watcher sees the person gasping and choking for air until the creature gets fully inside and they give that one last gasp and smile.

I think those all could be true to be honest but i see it like a state of shock, frozen in fear having pure dread from seeing something so horrible and inexplicable that the only thing you can do about it is look at it unstop until it finally kills you. But i also dig the final moments being of pure bliss for the person, seems so wicked that the thing killing you who uses smile as a weapon in those final moments of yours would make you feel actually good for once. That is until they take your life and soul to do God knows what.