Characterization
Prior to the crash of Oceanic Flight 815, Jack was born into a successful family, with aspirations of following in his father's footsteps. He attended Columbia University and graduated from medical school a year earlier than his classmates.
Despite Jack's gifts as a physician, he is haunted by his relationship with his father, a neurosurgeon at St. Sebastian Hospital. Jack exposes his father's alcoholicism, ending his career. In September 2004, Jack's mother, Margo, (Veronica Hamel) orders him to find Christian, who has exiled himself to Australia, and bring him back. Jack finds his father has died and goes to Sydney to identify his body and make arrangements to bring him home. Jack boards Oceanic 815 bound for Los Angeles on September 22, 2004, with his father's casket in the cargo hold.
On the island, Jack plays a key role in the survival of his fellow survivors in the immediate aftermath of the crash, instructing others to help those with injuries and using his medical background to assist the wounded. The survivors look to Jack as their leader; however, he is reluctant to embrace the position and repeats his father's rationale that he does not "have what it takes." Exhausted from tending the wounded, attempts to rescue drowning survivors and sleep-deprived, Jack begins chasing hallucinations of his father in the jungle. He drops his skepticism when he finds his father's coffin but it is empty. Struggling to cope with demands made by the castaways, he meets John Locke in the jungle, who provides some guidance for Jack's leadership. Jack returns to the beach camp and gives a speech on how they are going to live on the island and tells them about the caves; one part of his speech—"live together, die alone"—becomes a mantra of him and the survivors. Through his medical tending and visibility within the camp, Jack quickly develops many personal relationships with the castaways, most notably finding a potential romantic interest in Kate Austen, a right hand man in Hurley Reyes (Jorge Garcia) and a mutual respect for former military torturer Sayid Jarrah (Naveen Andrews).
Throughout the series, it has been stated numerous times that Jack is a natural leader. These things have been demonstrated many times by his ability to think quickly and analyze crisis situations. Jack intentionally represses fear and anxiety, usually in order to remain strong for other people, as he is the one they turn to during crises. Initially, he rejects claims of fellow survivors, such as Rose Nadler (L. Scott Caldwell) and John Locke, who believe they are on the Island for a reason and that the Island has mystic properties. He does this to not give his fellow survivors false hope. On the Island, Jack also seems to repress his feelings for Kate Austen, which he has only twice ever fully admitted to, and even then only once in a very emotional tone of voice. However, after escaping from The Island, Jack and Kate admit their love, move in together, and raise Aaron, Claire's son.
Jack's habit of repression sometimes does flare out, usually in his propensity to become violent when he is enraged. He is also prone to become highly obsessive and willing to do anything to help the survivors, even if it's to his detriment. Jack is, deep down, a very caring person and has sacrificed himself for his crash-mates several times. After being rescued, he sinks into alcoholism and drug abuse out of severe depression. He blames himself for leaving almost all the fellow survivors behind while he was safely rescued. This leads to his delusions that his father is still alive and a suicide attempt, which indicate that Jack has not been completely able to cope with what happened on the island. Digital Spy's Ben Rawson-Jones marked a "difference in the characterization of Jack has become known as the trustworthy, honest type since Oceanic Flight 815 crashed, so his blatant lies about the island under oath were definitely dramatic."
After being rescued, seeing visions of his father, and being visited by John Locke, Jack's outlook underwent a fundamental change. He began to believe in fate and destiny. When Juliet asked Jack why he had returned to the island, Jack told her "because I was supposed to." Jack's faith leads him to sacrifice himself to prevent the island's destruction. He believed that saving the island was what he was "meant to do." In many episodes, Jack is portrayed as having a Messiah Complex.
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