It's basically a circular thing. In Lost, time is a 'record' rather than something that flows continuously. Everything that they are doing now, in the 'past' causes the future events to happen or make sense. In the past, The characters can do all they want, but whatever they do has to fit in the constraints of the past. For example, they can't blow the island up, because if the island wasn't there, how did they get there in the future? They can, however, interact with the Dharma initiative, because who knows? Maybe what they do then will effect their initial landing on on the island.. (like them saving the baby, or richard eerily knowing what do to with Locke, the picture that Sun sees are all directly affected by what they did in the past)

Desmond is an exception because the show made him one. He's a victim of consequence and fate IMO