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Tobey Maguire suffers from back problems, which he aggravated during Seabiscuit. He responsibly warned the Spidey producers of this, and as a back-up plan they considered other actors for a worst-case scenario if, and only if, Tobey physically couldn't do Spider-Man 2. Kirsten Dunst, who has been dating Jake Gyllenhaal, lobbied for him, and he was definitely foremost in their minds as a fail safe. It turned out of couse that Maguire's back was much better by the time cameras started rolling on the sequel, so no alternate plan ever came close to being put into motion.
Tobey signed to do three Spider-Man movies when he was cast in the original flick. He renegotiated his contract after the mega-success of the first flick, but only as far as money. He is still very much signed to do the third film. Beyond that, time will tell.
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"Film is a disease. When it infects your bloodstream it takes over as the number one hormone. It bosses the enzymes, directs the pineal gland, plays Iago to your psyche. As with heroin, the antidote to Film is more Film." - Frank Capra