Your script depends on your ability to pull scenes together into threads, each of which helps the script. It helps to organize your scenes into threads, each of which has its own act breaks.
You’re most likely going to have a lot of little threads organized into bigger threads. Your biggest threads should be as follows:
Main Thread: This is the biggest thread of all, and is the actual plot. All of the action should happen here, as well as all of the big scenes. In essence, you should be able to cut out all of the other threads, and still have something. If this is the plot, all other threads are obviously subplots. At least half of your scenes should be part of this thread, and subplots should have about 10-15 scenes each (so a full sixty-scene script would have thirty scenes in the main thread, and two or three subplots).
Romantic Subplot: You need at least one romance, however contrived. Of course, it’s best if the romance can build naturally, with its own ups and down. This can act to add some interesting clashes that can be brought out in the main plot, as well as mess with other sub-plots. Try not to have more than one romance per script; anymore, and it’s more soap than story.
Character Subplot: This sub-plot is strictly to develop a character, fleshing out his personality and history, and should make the conflicts more interesting. Any character can be victimized by this sub-plot, even the villain’s henchmen; just have fun with it!
Running Gag: This is a joke that you have decided needs some building, from a small scene to resounding crescendo. Go lightly here; it’s tempting to have a number of them running around. You shouldn’t have more than two or three; anymore, and they become annoying rather than humorous. Also, you only need about five to ten scenes for a running joke; running gags are best used sparingly.
When it comes to threads, you can have scenes that are part of multiple threads. The ideal is to have each scene be part of its own thread, but sometimes it is more interesting to have a scene affect several threads; most commonly, the main thread and romantic threads will cross, as the romance hinges on the main plot. Nonetheless, do what feels right for you script; not all scripts are the same.
Keep in mind also that threads are what drives your script; each one strings through your script like strings of pearls, each pearl a scene that makes the next pearl possible. Thus, you can have weak scenes (and it’s good to have a couple, as they allow your audience breathing space). This is where you actually start building your script; these threads will coalesce into the script, with each one flavoring what happens. Just like recipes, combine the ingredients carefully, or you’ll make something rather unappetizing!