Why did Christine had to "act" the dead body on the beach while they could have used the real Arlena's unconscious body? They would:
- need less time
- not have to paint Christine brown
- avoid possible evidence (the bottle)
etc.
Why did they have to complicate it like that?
"Life is full of censorship. I can't spit in your eye." - Katharine Hepburn
The most important reason is that they didn't want to risk Arlene waking up, of course!
I think the idea in the original book is that Patrick was the only one who could kill Arlene because he was a strong man, so they had to create a false time of death to give him an alibi.
The movie kinda ruined it by having Christine knock Arlene unconscious. I have no idea why they chose to do that. Christine could just as well have finished her off. But I guess the idea is that Christine would never have been able to attack her like that on the beach. Arlene might've put on a fight and someone might've heard her. But Christine could never have dragged her body back to the beach herself, so she had to play her dead body.
They also needed clear bruise marks from the strangulation that would match a man's hands, definitely not Christine's
shareADDENDUM: Just rewatched it, and Patrick DOES make a big deal about the man-sized strangulation marks.
It apparently was part of their plan, after all, for Patrick to strangle her in order to rule out Christine.
I keep forgetting to add this:
Christine may not have been strong enough to move Arlena's unconscious body back onto the beach.
Better to knock her out in the cave and leave her for Patrick to kill with his big, large, definitely-not-Christina's hands.
Lastly, Christine could've gotten lucky and bashed Arlena's head out on the beach, but she'd still have "tanned" herself in case Arlena was stuck in the cave