Like many of you, I have been spending the prolonged off-season reviewing past episodes of LOST. I just watched the 23rd Psalm. You know, Eko's flashback episode. The one where he and Charlie go hiking thorough the jungle to find the plane full of heroin.
There's that great scene where Charlie is up in the tree and the monster comes right up to Eko's face. We get that 180 degree dolly shot of Eko's face through the black smoke, and we see images of people and things from his past in Africa.
It seems to be scanning him, in way similar to when it scans Kate and Juliet in season 3.
OK, so, here's the theory: What if the black smoke monster scans people physically and also scans their thoughts and memories? Then, it can take the form of that person, or someone from their memories.
That would explain the appearances of Jack's father, Dave and Yemi on the island. You remember, when Eko encounters Yemi. And, Yemi tells him to confess. Eko says, "I only did what I had to do to survive." An angry-looking Yemi replies, "You speak to me as if I were your brother!" Inferring that this apparition of Yemi isn't actually his brother, Eko asks, "Who are you?" But 'Yemi' turns away and vanishes into the jungle. Eko follows, repeating, "Who are you?"
Next thing we know, the smoke monster appears, picks Eko up, slams him around for a while and splits. So, I'm thinking the Yemi-image was the monster. It learned how to imitate Yemi from when it scanned Eko's thoughts in 23rd Psalm.
Let's take it a step further. The monster has had lots of contact with Locke. Ever notice how Locke seems to have two personalities? One is docile, helpless, ambiguous and confused. The other is heroic, strong, fearless and decisive. We go through phases where we think Locke is evil and others where we think he's a good guy. More than once, he is associated with black and white. What if one of the "Lockes" is the real John Locke who crashed on the island, and the other is the smoke-monster-imitation?
Think about it.