Alright,I had a thought a while back (before Season 6 started) that during Seasons 1-4, MIB was imprisoned within the cabin by the ash ring. Because of the ring, he could manifest himself as the (currently dead) people on the island..i.e, Christian, Yemet (anyone else?) but only visually. He wasn't fully corporeal at this point. He couldn't interact with the environment physically.
Something that argues against this is the various appearances of Walt, particularly to shannon...which we know for a fact he was alive at the time, and i'm really not sure what to think about that (unless it was just Walt's mad skillz!)
Now...at some point in Season 4, the ash ring was broken by someone/something (we don't know who/what). This allowed mib to be 'free' and allowed him to manifest himself fully corporeally anywhere on the island - 'Alex', 'Locke', Differently-clothed 'Christian' - more on him later... ).
As far as I can remember (correct me if i'm wrong), when 'Christian' guides Locke into turning the wheel, he is holding a torch. Similarly, when 'Alex' threatens Ben under the temple and again when 'Locke' kicks Jacob into the fire. Each form/manifestation that mib uses is able to interact physically with the surrounding environment.
So from Season 4 onwards, notably when mib takes the form of the deceased Locke and Alex, he is fully corporeal and therefore can perform his manipulation on Ben with more oomph (:D)
Now..as for Christian being clothed differently on these occasions... lets call these Christian1 and Christian2...creative, no?
Christian1 - This is the Christian clothed in his 'standard' trouser suit. We are left to assume that this Christian is the one inside the SEASON 1 coffin on flight 815. He's seen from Seasons 1-4. Examples are in 'White Rabbit'. He can't interact physically, he's a 'hallucination'.
Christian2 - This Christian is clothed in shirt + tie. He's seen in Season 4 and 5 at various points, mainly in or around the cabin. Correct me if im wrong, but don't we see him physically holding Claire's baby? He's also seen at the barracks with Sun and Lapidus (again, he picks up the photo frame i think?).
Now, people might ask: If mib is infact appearing as Christian, Why is it that when Jack discovers the coffin in the caves in S1, Christian's body is gone yet Locke's body is present when mib takes the form of him in Season 5+6?
Well, this might be down to the introduction of the alt-timeline. Could there be two different Christians from two different times, differently clothed? I believe one is dead on the island but which one is which? We only assume that the Christian that is supposed to be in the coffin in S1 is the one in the trouser suit. It probably is of course, but could it be vice versa?
This would help explain the seemingly random disappearance in 'LA X' of alt-christian's body & coffin...
Thanks for taking the time to read this (I understand I may have not worded this entirely correctly - im tired :p). Comments, thoughts and criticism are welcome!
Cheers