A spec script, also known as a speculative screenplay, is a non-commissioned unsolicited screenplay. It is usually written by a screenwriter who hopes to have the script optioned and eventually purchased by a producer, production company, or studio.
Many spec scripts go on to win Academy Awards. Callie Khouri sold Thelma & Louise to Ridley Scott’s production company for $500,000 in 1990; Matt Damon and Ben Affleck sold Good Will Hunting to Castle Rock Entertainment for $675,000 in 1994; and Alan Ball sold American Beauty to DreamWorks Studios for $250,000 in 1998. 
In 1933, Preston Sturges is believed to have sold the first spec script in Hollywood history. Fox bought 'The Power and the Glory' for $17,500 plus back-end revenue. The movie went on to tank at the box office. 
Originals were not smiled upon in those days, believe it or not. There was very little interest in originals in those days. [...] Studios, distributors wanted the assurance of someone else having thought a property worth publishing[...] In those days, if you went to a party in the Hollywood community and somebody would ask, "What are you working on, Ernie?" and you replied, "I'm doing an original now," the response would be "Oh." [...] Like they were a little embarrassed [...] If you were working on something that you were going to create all by yourself, they'd secretly think, "He's in bad shape. Working on an original." That definitely was the climate at one time in this town.
In the late 1960s, William Goldman sold his spec script Butch Cassidy and the Sundance Kid to Warner Brothers for $400,000 in a studio bidding war. The script went on to win the Academy Award for best screenplay. This event precipitated a rise in screenwriters writing on spec. 
Attracting producers 
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If the writer of a spec script has an agent, the agent will identify a number of prospective buyers who may range from small independent producers to executives working in the major studios, and attempt to build up 'heat' under the script. The script may be sent out simultaneously to all the prospective buyers in the hope of attracting a bidding war.
If the script sells, the writer may receive a payment of anything from a few tens of thousands of dollars to several million. If not, the script is sometimes dead in the water because it is now in the databases of the studios and development executives, and has been marked as having been 'passed' on.
If a spec script is not picked up, but the script is considered good, the screenwriter may be offered a writing assignment. Spec scripts are often written by unknown screenwriters looking to prove their storytelling ability and make a name for themselves in the film industry.
Sample script 
Sample scripts are not intended for production but are used for showcase purposes only. Unsold or unproduced screenplays often go on to be sample scripts.
See also 
- Screenwriter's salary
- WGA script registration service
- Dreams on Spec - a documentary on screenwriting
- The Black list - an annual survey of Hollywood executives' favorite scripts
- Vanity Fair, "When The Spec Script Was King" by Margaret Heidenry, March 2013
- John Brady, "The craft of the screenwriter", 1981. Page 204