plot explanation - Cracks in the wall inconsistencies

In the 11th
doctor's 1st series there is a story arc involving cracks appearing across the universe
and removing people from history.

However I don't
understand, in some examples people are removed from history (the troops from the
Byzantium and Rory), however in other places beings appear to be able to enter and exit
the cracks at will (Prizoner Zero and the Doctor when checking for debris inside the
crack).

Is there any opinion on why the cracks have
different effects on different people?

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class="normal">Answer

The general
suggestion (at least as far as The Doctor is concerned) is that as a creature of time he
is able to control his contact with the effects of the crack. He has to give himself
over to it willingly before it consumes him.

In the 11th
season, the nature of the crack is said to not be fully understood and will be effected
by the "Great Unasked Question" of season 12. It is possible that the Silence and the
race that Prisoner Zero is from are involved in this greater issue and can transcend the
effects of the crack.

We do see that once inside the crack,
the Doctor has some general control as he steps backwards and 'unwinds' through his own
history.