Candidates have to be dead before MiB can look like them. Everyone else is fair game by Darkhunt

Many people have wondered why some highly implied apparitions that could be credited to the Man in Black were possible given his dialogue with Jack in which he states John had to be dead before he could look like him. These include but are not limited to his highly plausible appearances as: Isabella, Emily Linus, the Altar Boy, the Drug Lords, Kate's Horse, Dave and Taller Ghost Walt. The issue that has arisen is that each of these characters are either dead or their corpses were not on the island for the Man in Black to imitate.

Now I know the Man in Black is a world renowned liar by now, but his talk with Jack was doubtless an expositional scene in which the producrs could explain some of his earlier interactions in the story and the nature of how he came to taking John's form. Many theories have to cop out to say that the Man in Black was lying, but what if he didn't?

We know Lost well enough by now to not look at dialogue at face value. I think Ben's quote from This Place is Death is very appropriate "That's right, Jack. He didn't go to see me, I went to see him."

In the same fashion, Jack didn't ask if The Man in Black had to be dead before he could look like people. Jack asked if the Man in Black had to be dead before he could "look like him." Him being John Locke. The Man in Black acknowledged that as true, but didn't explicitly state -all- people needed to be dead before he could look like them, and was probably why he dodged Jack's question when he asked "who else did you look like".

It's been well established by now that there are "rules" protecting the candidates from The Man in Black, as long as they are alive that is. What if one of the rules is that The Man in Black can't take the form of a candidate? In the case of John Locke after all, when he's dead, he is no longer a candidate, his name is crossed out. This was the only thing stopping the Man in Black from taking the form of John Locke.

So my theory is that all of the other characters that the Man in Black has taken the form of are simply possible because they are not candidates. None of them are, not even Walt. Death isn't a prerequisite of the Man in Black's shapeshifting but simply falls into the equation. The Man in Black can obvioussly learn about these minor characters by scanning the minds of the major characters (as he did Richard). By knowing what a character looks like he can shapeshift into them as long as they aren't a candidate. Again, Jack was "kind enough to bring Locke's body back there" which allowed MiB to scan Locke's memories since his death, which was vital for his interactions with Ben. So he didn't need Locke's body, he just needed his thoughts.

In the mobisode "So it Begins" we see the first appearance of Christian where he tells Vincent to "go find his son", knowing where he had woken up in the bamboo forest. It's heavly implied that The Monster scanned Jack prior to this scene. Many people wonder the significance of why Christian's body wasn't there whilst Locke's had to be, but again, it's probably that Christian wasn't on the island to begin with in a similar way to Isabella. His corpse probably fell in the water during the impact of the crash, meaning that the Man in Black never used Christian's body in the first place as he did Locke, and this was still possible because Christian isn't a candidate.

For people that still have doubts and think Christian's body was actually in the caves before the Man in Black interfered (or in some cases, that Christian was reanimated), there's a screenshot on this page of Christian's white tennis shoes during the first 5 minutes of the pilot http://lostpedia.wikia.com/wiki/White_tennis_shoe If those shoes managed to go flying from the casket during the impact of the crash mid-air, then something most definitely happened to the body aswell... therefore we can concede that Christian's actual body was never in the caves and that the body played no part in MiB taking on his form.

Another character that hasn't been brought up much is Claire. During Season 4 we see an apparition of Claire (despite her being alive) off island telling Kate that she can't bring Aaron back. If Kate had brought Aaron back, then that would have foiled The Man in Black's plan for Claire to help raid the Temple and chances are she would have won back some of her sanity in the process. Claire wants nothing more than to see Aaron again, so whoever that was, it definitely wasn't her. Again, in "The Package" the Man in Black states that Claire is not a candidate, and thus he can take her form without her being dead, and with the intent at hand I don't see how "the island" or Jacob would disapprove. We've also seen the Man in Black off island as Christian (debatable, but it's what I believe despite the discrepancies).

So with all those points in hand, I think this is the best stance we can take in explaining why some of Man in Black's appearances seemingly contradict what the Man in Black has told us about his abilities.