I'm beginning a new spec script and outlining it. Some writers don't like to work from an outline. I find it helpful for the first draft or two. I use it simply as a reference. I'm not rigid about it. I find it helpful to write "notes" before I start a script -- little phrases, key scenes, character quirks, slang, etc. Then I put them in groups -- so if I need slang to fit within my narrative or dialogue, I have a list to choose from. I did this when I wrote the softball spec script and needed various sports terms and catch phrases.
I also think it's helpful, for me, to break the Three Act structure up so I see visually where I'm going from one act into another. Simple beats. I don't get crazy over it. But I do find that it's important to lay the ground work -- to me, it's like anything that you're starting from scratch -- cooking, baking, building a piece of furniture, anything you create -- it saves time if you know where you're headed. The final result may still surprise you, but an outline helps to get you on your way like a map.
Even with maps, we still get lost -- too bad they haven't created a GPS for scriptwriting. Not yet anyway. I'm sure some Hollywood guru is working on it.