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It seems to me that most movies are director driven - ie powered by the directors ego and own artistic creativity - examples - James Cameron, Quentin Tarantino, Christopher Nolan etc., or by well established directors searching out screenwriting talent - such as Clint Eastwood and Unforgiven, which, I believe, was optioned for ten years or so.
Then you,ve got experienced Producers - Tom Hanks, Spielberg, Bruckheimer etc. who have their own ideas, as well as probably being inundated with qualified, creative pitches and ideas from the top echelon of screenwriters and agents.
I may be wrong, but I've yet to spot a blockbuster written by an unknown spec screenwriter that has sent their screenplay to a screenplay competition etc, done well, and then had their screenplay turned into a big movie.( Please tell me if you know of one!)
Shane Black was a breakthrough screenwriter, although none of his work has impressed me, it did impress Hollywood, and for a time he was the highest paid screenwriter in Hollywood, although ironically, most of his finished movies were extensively rewritten by the studios.
Many times the studio will chase the work of a well known writer - ie Jurassic Park, Disclosure, or John Grishams work etc.
I believe Michael Crichton was paid $5 million for a screenplay (or the rights?) of Prey before the book was even finished, but I haven't seen the movie come out?