How do you tell when a screenplay is taking itself too seriously? What do you do to correct it?
Well, it's a bit difficult to gauge exactly what you mean when you say that the screenplay is "taking itself too seriously" -- do you mean that you get the sense that it's a bit too preachy or something?
A lot of times you, the writer, have something that you want to get off your chest and you're in a such a hurry to say it that you will sometimes shove your sentiments directly into the mouths of your characters, essentially hijacking them and forcing them, against their will, to say what you want them to say about "whatever it is" rather than doing what they want, which is to go about the business of living their lives and pursuing their own objectives within the world of the story.
This is what people really mean when they say "show, don't tell" -- they're not talking about using visuals rather than dialogue. They mean you, the writer of the piece, should convey whatever it is that *you* have to say, through the machinery of the narrative -- through action and drama, not through making characters say and do things for the sole purpose of conveying your particular agenda directly.
When your characters, by sheer chance, happen to share your deeply held views on a particular subject and also, by sheer coincidence, feel the uncontrollable need to tell those around them, in the world of your story, all about it, at great length, then just maybe there's something that needs to be re-thought.
Of course, I could be completely barking up the wrong tree and you might have something totally different in mind when you're talking about your script taking itself too seriously.
NMS