Choose the novel you want to adapt to the screen or stage carefully. The heavier the novel is on dialogue, the easier it will be to turn into a script. In many cases adaptations simply lift whole sections of dialogue and place them into the script. A novel that is mostly narrative, revealing much of what happens through the thoughts of the characters, will be much tougher to adapt. In addition, think carefully about the setting; a novel that takes place mainly in the interiors of modern homes or buildings will make for the easiest set creation; conversely, a book that has as its setting a location in a foreign country hundreds of years ago presents a huge investment for the filmmaker.
Write out in detail your vision for the play or movie so that you will have a reference point to check against as you work. State in specifics what you see as the theme for the work, what the mood or tone will be, what points you want to make, what you want the overall look of the piece to be, and, if you choose, who specifically you see as the audience for the adaptation. In addition, think about how closely you hope to imitate the author's intentions for his or her work; ask yourself whether you will stick to the author's viewpoint and vision or make it your own and, if so, why.
Determine how much of the plot will be included in the adaptation and write a simple one- or two-paragraph description of the action. Begin by deciding, in the case of a stage play, what moment will mark the end of the first act and divide the plot events accordingly. For a film, it's necessary to look at the projected running time and to figure out how the action will be divided throughout. Subplots can be added only when it's determined how much time can be spared from the main plot.
Concentrate on the characters by first determining which ones are the strongest and which ones, if any, can be eliminated. Write, just for an exercise, a monologue for each character. Do this so that you can bring the characters alive for yourself. Include comments on such items as each character's lifestyle, hopes and dreams, likes and dislikes, fears and opinions. Then, for two or three central characters, write another monologue in which they comment on their attitudes and relationships with one another.
Begin writing the adaptation by getting key scenes down on paper. Some playwrights and screenwriters like to write the last thing the audience will see and hear first, to give a sense of direction for writing the rest of the script. Others choose to write each of the key scenes when major dramatic events take place. These are then put in order and the writing is done to string each of these together. Sometimes the opening scene in a play or movie is written first because it sets the tone for the rest of work. These scenes are often rewritten after the script is finished.