In preparing to write my stage play, "The Sexiest Man Alive," I read three fine books on the craft: The Art and Craft of Playwriting, Naked Playwriting (terrible title, fine book) and In Their Company (interviews with 50 iconic playwrights).
I took eight pages of single-spaced notes but here are the nuggets I most want to remember:
Most memorable characters are likeable, even the villains, who you admire for their brilliance, soft spot, etc.
Write for people smarter than you. Make your characters smart, if only street-smart.
Make your audience eager to know what will happen next.
Your protagonists must have arcs: the play's events must transform them.
Always keep your hero in trouble. We must see the characters under pressure, struggling, and see how they respond.
In addition to the big conflicts, a play needs dozens often hundreds of smaller ones, each resolved and replaced with another often bigger one.
Your audience must feel delight when the power shifts from protagonist to protagonist and back again.
Hitchcock described suspense as "the addition of information." Like a strip tease: a little more, a little more.
A full-length play needs more than one question to keep the audience interested. So, pose sub-questions, inner questions, side questions, thematic questions.
Write sparely. Better to show than tell. Only have your characters speak when they are compelled to.
Real characters interpret, misinterpret, and read unintended meanings into everything said to them.
Sometimes, the speaker is evading, pouting, defensive, talking to themselves as much as to their protagonist.
Constantly ask yourself not just what would your character do but what could they credibly do?
Subtext-laden dialogue (not saying what you mean) is high dialogue and fun for the audience. It's often accomplished by understatement, inarticulateness, or metaphor.
Look for opportunities to create honest spectacle. For example, I may have the husband dump a bucket of ice on his wife's crotch.
Perhaps most important, you must bring to your playwriting, all your heart, soul, humor, imagination, sensibility, history, and your life's experiences.