We could say that free will doesn't exist in Lost, or at least there wasn't a character completely free to make their own choices. If there was, the character would necessarily have knowledge of all the island's mysteries and forces, something that even Jacob seemed not to have. So what follows are some thoughts about the role of free will being very limited. They seem very random and inconsistent, but i hope in the end it can make some sense.(And forgive my poor english!)
The human being is the result of many influences and determinants. Not only social factors but also biological. All these combined, as much from the past as from the present, give rise to who we are today. The "flashbacks" were used to we understand better the Lost's characters, in the present. Our parents, the kind of relationship with them, are certainly a key part of what we are made of. Parents, the people that raised us and with that transfered a series of good, or bad, social values; the people who gave us their genes and thus possible genetic predispositions. Parent issues were a big deal for our Losties. But as we grow up, increasingly are the influences and determinants that shape who we are and who we'll be.
For Sawyer, the father's suicide following his mother's homicide determined the sardonic con-man he would became. Fatherless, motherless and with a huge thirst for revenge that carried him down a wrong path. However, most likely, due a combination to all influences and determinants that he found, it couldn't have been differently. In Sayid's case, he only became a torturer after being manipulated to be so by Kelvin Inman. The foreigner used victims' images to induce revolt and desire for revenge in Sayid. Through Kelvin, the determinant, a man who swore to protect his nation and his people, met torture and saw it as a way to keep his word. If could they have chosen another path? At one point, a man does not choose another path if he can not see it. But with time, new influences and determinants appear, people change and the paths change as well. For Sayid, when he first see Nadia again (still Iraq) was another determinant.
Lost began not long after the U.S. invasion of Iraq. We heard much about tortures at Abu Ghraib. In the last 7-6 years, dozens and dozens of Iraqis die every day, thousands of young American soldiers died far from home. When someday in the future, the Lost universe be examined in detail, people won't fail to talk about the historical and social context at the times of its creation. Like in fiction, were all the soldiers in full use of their free will, knowing the implications, history and culture, economic disputes and power struggles, and complications, when they decided to enlist? One of my doubts about these series is whether the characters actually died for nothing. That's also the reason I have nicknamed the flash-sideways of "hope." People's "hope" of their deads being in a better place.
Returning to free will, it is not absurd to think that Jacob carefully and coldly manipulated Losties' lives for not only one of them replace him as well, and this was perhaps the most important, for they to want kill MIB. Locke, Richard, Bea Klugh, among the Others, so many people who blindly followed Jacob and believed in the island, who wouldn't think twice on replacing him. If all that was needed was a candidate accepting the job, why not Locke? The one who agreed to give up his live to protect the island. The truth is that Jacob decided to appear to Losties just when they were quite determined to kill MiB. Although, a new Jacob (the substitute) being enough to control MiB and prevent him from leaving the island.
"And as a result, choices you thought were made, were never really choices at all. He was pushing you,(...)."
Free will? Jack was always the "fixer", who carries the humanity behind his back, the man with the god complex, at least since his father told him "You don't have what it takes". The Jack that was given to us to know certainly would try to save the island in an attempt to save his friends. There was no other choice or another way. That's who he was. No, he didn't exercise his free will, at least not fully. The combination of all influences and determinants of his life made him a man who sacrifices himself for others, for Jack there was only one option. To have more choices to help him save his friends, he would need more knowleadge about the island's mysteries. Anyway ...
... free will was never the main theme of Lost, nor destiny. More important was Jack forgiving himself, and consequently, finding his redemption. Lost, Found and Free. "Each one of us was brought here for a reason." Jack found it, or believed he found it, and for him was enough.
"Whatever Happened, Happened" and "Let It Go", these might be the greatest lessons of Lost. Looking into the past, review all these influences and determinants, and realize how much --really--- could have been done differently, not in a wrong way. You can not change the past, but "everything changes" and perhaps yet we can find ourselves.
Thanks for your attention!