Before you just 'shoot off an answer to this, read the post below:
Just suppose that Jacob cannot time travel, but he can read minds. (Not too big a stretch; Miles can read the minds of the dead; the smoke monster appears to 'judge people's actions' by reading their thoughts).
All of the candidates (Locke, Reyes, Sawyer, Shephard, Jarrah, Kwon) went time-travelling. Locke, Sawyer and Kwon possibly as far back as 1954, but certainly to 1974, and Jarrah, Shephard and Reyes to 1977. This means that Jacob would have been alerted to the existence of these people as early as 1977; his visit to Sawyer in 1976, could have been as a direct result of them coming to the island. The Candidates by their own actions introduced themselves to Jacob.
This provides an interesting paradox that I have not seen discussed anywhere. Jacob may not have picked the candidates out for scrutiny. They arrive unbidden on the island from the future, and he then reads their minds, to find out about their histories. He gathers further information on them via the Lighthouse.
He knew exactly where to be to make interventions in their lives because he had seen those incidents in their memories. The question then becomes whether any of those interventions had any effect whatsoever, or was Jacob just doing what he had seen himself doing in their memories. Is Jacob governed by what happened, happened, or could he have chosen not to do what he did? In fact, did he do something different to what he saw?
In other words, in matters of time travelling, does Jacob have free will?