Body Of Evidence

There will be Lost spoilers going up to the last episode aired:

The season 4 Lost finale, where Locke is advised by Christian Shephard to move the donkey wheel, raised a lot of speculation because even though Locke had broken his leg, Christian said he couldn't give him a hand. So people guessed Christian was incorporeal.

But looking back, I think it's something else. First, we now believe that Christian, not to mention appearances by Alex and even Locke, and Smokey himself, were likely covers for Jacob's nemesis (we'll call him Esau).

Both Jacob and Esau seem to manipulate people. It appears, in fact, that, for whatever reason, Jacob brings people to the island. And Esau tells Jacob that he had to go to a lot of trouble to find the loophole that allows Ben to kill Jacob, and we see a lot of the maneuvering he's done to get us to that point.

But there's no question Jacob supports free will, as he makes it clear to both Hurley and Ben that they have a choice in what they do. Meanwhile, Esau wants to kill Jacob but apparently can't do it directly. So maybe the same "rules" apply to Esau--that he must allow free will. He can prod and strongly suggest people do what he wants, but he's got to allow them to make a choice. So these characters he plays do have physical dimensions (Smokey sure seems to, and Alex slams Ben against a wall), but Christian is simply not allowed to physically help Locke--Locke's got to do it himself or it doesn't count. Just as Esau (as Locke) could go into the sanctum sanctorum, but couldn't himself do the killing of Jacob.

Also, it seems very possible when he appears as these others that he retains some of the qualities of these characters. Locke had no doubt changed, but he still would say things that sounded more like Locke than Esau. And Esau and Christian shouted at Locke "say hello to my son" before he left. It was possible that was Esau ensuring that Jack would believe Locke, but it seemed more like an afterthought that the real Christian Shephard would add.