You gotta plan...yo.

[YoshisIsland3]

Why plan when you can just make everything up as you go along and hope for the best??

For one side of the argument, you could plan so that whatever comes along next, won't be a total wreck, when you make it. (Level, event, etc.)

For another, you could just make everything as you go along, but it won't be as smoothly as planning out what you want for the best.

It all depends on the person and how they develop it.