Ian Somerhalder made his name playing Boone in ABC's iconic mystery series, Lost, but the original version of the character was very different, and suffered with schizophrenia. Boone Carlyle was one of the original Oceanic 815 survivors, debuting in Lost's very first episode. While he may have been a heartthrob for viewers in the real world, the on-screen Boone only had eyes for his step-sister, Shannon, and this relationship informed much of Boone's character in the show's early episodes, with the duo's jealousy and dysfunction coming to the fore thanks to the island's isolated setup.
Having previously been a spoiled rich kid on the outside world, Boone was not naturally suited to survival on a desert island, but eventually fell under the wing of one John Locke. Working together, Boone and Locke uncovered the hatch leading to the Swan station and made first contact with the tail section survivors. It was during this mission, however, that Boone fell to his death. The ramifications of Boone's demise were felt throughout Lost's remaining seasons, and acted as the trigger point for the ongoing struggle between Jack and Locke.
Despite his importance to the plot, Boone was originally envisioned as a very different character. According to the Lost bible that was compiled by J. J. Abrams and Damon Lindelof after the filming of the pilot episode, Boone's backstory as the wealthy heir to a successful business was always part of his makeup. However, the description makes no mention of Boone's romance with Shannon, and instead focuses on mental health issues. The document states that Boone was secretly diagnosed with schizophrenia as a teenager, and managed the condition with therapy and medication. Deprived of both on the island, Boone was supposed to gradually lose his sanity as Lost progressed, and would be desperately trying to keep his condition a secret from Shannon, who was unaware of her step-brother's illness.
Although Lindelof has since downplayed the bible's contents as mostly made up to convince ABC to take Lost to series, it does line up with the already-made pilot. The first time viewers see Boone, he's incorrectly performing CPR, despite having been a life guard previously, and he then rushes around collecting pens for Jack Shephard, trying to help out, but doing so in a slightly erratic manner. While the audience is left to assume Boone is simply still reeling from the shock of the crash, the original schizophrenia angle casts the character's first scenes in a new light, and could account for both his medical mistakes and his later outburst towards Shannon.
Boone's mental health arc may have been dropped to accommodate his romantic storyline with Shannon, since this isn't mentioned at all in the bible and Sawyer was primed as a love interest to Maggie Grace's character instead. Alternatively, Lost's producers may have realized the similarities between Boone's medication withdrawal and Charlie's heroin withdrawal. While obviously very different scenarios in the real world, these two stories would've felt very similar on TV, with both Boone and Charlie deprived of drugs on the island and spiraling out of control. Interestingly, Charlie's addiction is included in the bible's description of the character, so Boone's schizophrenia story wasn't ditched and reworked into Charlie's substance abuse, they were always separate plot points.
Lost fans will remember how Boone ultimately became a devout protege to John Locke, even going so far as to take a hallucinogenic trip at his mentor's behest. It's possible that the character's schizophrenia would've played into this development, as the other islanders are already suspicious about Boone's decision to follow the strange and mysterious Locke. The struggling mental state of Ian Somerhalder's character would've painted Locke as more manipulative and misguided than the final version of the story - someone who would take advantage of a vulnerable person in order to further his own ends.