Absolutely
always.
In fact, the screenplay is the LAST thing I do.
First, I get an idea. Usually this idea, if I'm lucky, comes with a few plot points, endings on the side. I develop these into a bullet-point of the story.
Then, I fix these bullet-points around until its perfect and examine the themes, characters, why something happens/someone does this or that, THEN I write the full plot. Sometimes they range from 2 to 7 pages in 12pt font.
With this in hand, I divide the plot into thirds. I ruthlessly edit each and every one of them and then split the thirds into scenes.
Now, I make a bullet-point outline of every single scene in the film that I have in mind. I handwrite on legal pads/notebooks the screenplay (dialogue, action, scene headings, everything). The dialogue I first write freestyle on paper (almost shorthand) in order for me to pay attention to it being fluid.
With my handwritten screenplay, THEN, and only then, I type it up.
The bottom line is that writing a screenplay without a concrete plot is, at least in my opinion, almost foolish. You need to know where your script is going if you want it to be a nice, clear narrative.
My two cents.
