5. Live. Die. Repeat
Rubin admits that in his first drafts - or at least in the planning stage, which ran a lot longer than the writing itself - he only had a high concept, which said that the main character was "the only variable in the universe."
A time loop was included in which the character was forced to relive the same day over and over. From there he built the script around the core idea that one lifetime just isn't enough - hence the Buddhist appeal.
"There are some people, those arrested development-type men who can't really outgrow their adolescence. And I thought, 'Well, maybe one lifetime isn't enough. Maybe you need more."
He also started his story in the middle, with Phil already in the loop and explaining in voice-over how he got there. Ramis thought it was better to show the first day.