ABC's Lost was one of the biggest shows on television during its run, and it's hard to imagine anybody other than Matthew Fox as Jack Shephard. When the show was in the early stages of development, however, Michael Keaton was offered the part in J.J. Abrams' new series. Ultimately, Keaton passed on playing Jack, and he has this to say about why he chose not to join Lost:
J.J. and I had a conversation; he told me about this thing he does...He said, and here's what happens, the guy that you think is the lead, dies in the last 10 minutes. Those kinds of things intrigue me, so I went, 'Oh.' And I thought, 'Yeah.' The idea of doing an hour television show--I was just too lazy. So, I thought, wow this is pretty good, but then I don't have to be in the series. I think what happened was--and I've never really talked to him about this--[either] he thought better of it or the studio said, 'That ain't gonna happen.' And then there was kind-of a half a conversation about, 'Well, do you have any more interest.' The offer changed.
Lost would eventually run for six seasons from 2004 - 2010, with most of the filming taking place in Hawaii. If Michael Keaton had signed on for a starring role on the series, he might have been playing one character for quite a while, and he wouldn't have been able to work on many of the films that kept him busy over the years. Of course, the show might not have lasted as long as it did without Matthew Fox as leading man, but Keaton doesn't seem to have any regrets about passing on the project.
If the pilot for Lost had proceeded as originally intended by J.J. Abrams, neither Michael Keaton nor Matthew Fox would have had the chance to play Jack for more than a single episode. The original pilot was scripted to kill off Jack after spending most of the episode building him up as the protagonist. The idea was to make sure that viewers understood that nobody is safe on Lost, no matter how important the character may seem. Kate was then slated to step up and assume the leadership role on the Island.
J.J. Abrams was eventually talked out of his plan of killing off Jack by ABC execs, and Lost became a very different show than it would have been if the smoke monster had munched on Jack in the very first episode. The twist of killing a Jack played by movie star Michael Keaton in the first episode would have certainly gotten people talking, but the show was probably better off for letting Jack live. Keaton's comments in THR's Awards Chatter podcast indicate that he just wasn't the right fit for Jack's character after the script tweaks.
We can only wonder now what Lost would have looked like if J.J. Abrams hadn't changed his original vision for the pilot. If Michael Keaton had played Jack and been killed off, we would have gotten to see how Kate would rise to a leadership position (and we would have been spared Jack's awful tattoo episode). The show turned out to be a mega hit that would launch the careers of some relatively little-known actors, and Keaton isn't exactly lacking for work nowadays, so it may be for the best that things turned out the way they did for Lost.