Who says you need to stick with rigid plans to make great art? Even when there's millions of dollars in the balance, it's sometimes better to just wing it. After all, the best-laid plans of mice and men often go awry.
There's obviously a rich history of films with poor preparation ending up the turkeys everyone expected them to be from the outset and of projects that scrap their initial plans and make it up as they go along with rewrites and reshoots ending similarly, but for all of them, there are major exceptions.
Even films that have been years in gestation with a script locked down for just as long can end up being terrible. So it's not always the worst idea to gamble and just go with the flow, particularly when it can lead to such impressive results. Fail to prepare and prepare to fail? Not always, know-it-alls...
Given how terrible the title is (especially compared to Live, Die, Repeat, which they basically used for the home release unofficially), it shouldn't come as too much of a surprise that not a lot of forethought went into Edge Of Tomorrow's production.
Apparently, the film began production in 2012 with only a rough outline of its story and the action sequences (which admittedly were both stunning). It was based on awfully named Japanese novel All You Need Is Kill, but Doug Liman clearly didn't need a full script when he started, which producer Erwin Stoff said was "not the most calm-inducing experience."
Still, the final product is incredible - and one of the best sci-fi action movies of recent times - so maybe Liman ought to start more projects without a script and rely on his directing.