This is one reason why many people say, just write. Plan to whatever degree you wish. At some point, you'll start to itch. That's when you start writing. Maybe the plot's not fully outlined. Maybe the characters are still rather hazy, and the landscape's dimly seen. As you write, you'll fill it in. And out.
I'm sure some of the missing details will be discovered as I write. But I do feel I need a very good idea of certain things before I begin. All these things affect the prose, the voice, etc., as I write, and help me focus on writing, on "being there." At least, I feel more comfortable the more I know before I begin to write.