Don’t assume that things will work out as you write your script. Plot demands serious thought and you will just end up wasting time if you skip it.
For me, it boils down to having to learn early in the writing process what works and what’s not and not jump at every idea you have, especially if you have a lot of them, which I have all the time… Endlessly…
Basically; don’t start on something when you got nothing.
The Horrors of Writing 