When you decide to write a spec script for television, you must first decide the series for which you want to write a sample episode. Focus on series you like, that fit your writing style, and that are likely to be renewed for at least one or two more seasons (because the ratings are strong and the contracts of the stars aren’t close to expiration).
Rent, stream or purchase the DVDs of the previous seasons of your chosen series, and record whatever current episodes you can. Watch the episodes the first time just as a regular viewer, to experience the emotion in a normal way.
With the second viewing, take extensive notes for each episode in the form of a step outline. A step outline is simply a list of the events that occur in a story, in order.
When you have completed step outlines for at least four episodes, look for the similarities among all the episodes. This will show you the requirements and structure for the series. Your sample episode must then conform to that pattern (including where the commercial breaks occur).
For example, repeated viewings of The Good Wife reveal that in every episode: Alicia is the hero; she is brought in to help with a new case at the law firm; scenes involving this case will alternate with scenes involving the other regular characters (Alicia’s co-workers, family, and ex-husband); the mystery surrounding Alicia’s and the firm’s client will be resolved by the end of the episode; and the story line involving her personal life will remain unresolved, but will have reached some new turning point. Often Alicia’s case for the episode will mirror her personal life in some thematic way.
Since all of this is true for almost every episode of the series, your sample episode must also include every one of these elements.
Never take it upon yourself to write an episode that will change the “rules” of the show or the course of the series. The producers of each series will decide if and when they want Alicia to murder her mother in law, Haley and Dylan to get married on Modern Family, or Don Draper to come out of the closet on Mad Men; don’t you write those episodes.
Don’t worry that the elements of the series will have changed by the time you finish your screenplay, even if you’re writing a sample episode of an open ended series like Castle. Your script should be consistent with where the show is when you submit it. The agents and executives who read the screenplay won’t expect you to be able to predict the future – they only want to know if your writing is both original and emotionally compelling within the parameters of an existing TV show.