Robert McKee is a screenwriting guru and the author of Story: Substance, Structure, Style and the Principles of Screenwriting.
Research. Writer's block is not a paralysis of creativity but a malnutrition of material. Talent is a beast that feeds on a writer's memories of first-hand experience, observations of life, and knowledge of how the world works. If your imagination has fallen silent, it's because you're starved for something to say. Therefore, stop pounding your head against the keyboard and fill your mind with facts and insights. Ask yourself three key questions: "What do I know from my own life that parallels my characters' lives? What have I witnessed in other peoples' lives that echoes of my characters' conflicts? What nonfiction books have I read that would shed light on the time, place, and society of my story's setting?" If the answer to those questions is "nothing," the reason you're blocked and the solution to the problem will be obvious.
Strunk and White's axiom, " Omit needless words" always inspires me. Every time I pare a complex sentence to its fewest possible words I discover a core of meaning that propels; the paragraph. When I strip down paragraphs, chapters focus. The same is true for making dialogue drive scenes and scenes build acts. Go lean. Overwriting blurs creativity.
The best advice my writing professor gave to me I pass along to you:"Convert exposition to ammunition." Your characters know their world, their history, the other characters and themselves. When writing dialogue, let them use what they know as ammunition in their struggle to get what they want. Don't write "Mary, how long have we known each other now? Must be over twenty years, right? Ever since we were in school together? Girl, that's a long time." On-the-nose dialogue like that always feels phoney and stops a scene dead. Instead,insert conflict and convert those facts to ammunition: "Mary, for God's sake blow your nose and stop crying. Girl, you are the same weepy child you were twenty years ago in school. Time to grow up." The audience's eye will jump across the screen to catch Mary's reaction while it indirectly learns the character history it needs to know...and the scene flows.