Contrasted from novels
Novels as well as short stories are first and always words on pages. Offsetting the disadvantage of no visual/aural presentation is the capability of language to express thought. Unlike novels, screenplays and stage plays cannot get into their characters' heads, except by implication of speech and behavior. There are also the lack of a narrator's insight, provocative observations, and the alchemy of of skillful narration. Movies and plays are dependent upon the combined artistry of sight and sound to carry a scripted story.
Screenplays and stage plays also generally differ from novels in tightness of story form structure. Novels commonly are bookmarked, closed at chapter's end, and the next day opened where a reader has left off. This discontinuity of experience allows for a looser knit. Rate of reading as well as rereading to absorb elaborate descriptions, to consider sub-plots, to reflect on passages of commentary has the reader set storytelling pace rather than the author. This can be helpful in reconnecting with a novel from one day to the next.
A movie or play, by comparison, is experienced in one sitting. Viewers see a drama uninterrupted from beginning to end. Tightness is incumbent upon story structure to keep boredom at bay with an audience held captive, in effect, for a couple of hours. A play or movie must always, well, move to keep its audience connected to what's on stage or screen.
Contrasted from each other
Compared to movies, plays can be described as all in "master shot," a single visual perspective framed within a proscenium arch. Relative simplicity of presentation is in even long plays being staged on only one or very few sets.
The progression of drama in stage plays relies almost solely on dialogue exchanges rather than the mix of dialogue and action in motion pictures. (playwrights usually are accorded the most creative credit in so-called legitimate theater, whereas film is generally considered a director's medium).
Location filming, camera angles, computer graphics, editing, etc. give a more true to life feel in movies than plays. The confinement of the stage, however, breeds its own suspension of disbelieve, in effective use of language, set design, and lighting.
On marketing screenplays and stage plays
Conventional wisdom for those whose writing flows even on the outer edges of mainstream, is to design what can be an often elusive balance between:
What's comfortably familiar in a story.
What's intriguingly fresh about it.
In addition, it's essential to get readers' interest within the first few pages and build it to the end.
With screenplays written for a mass audience, it's a good idea to choose a genre in vogue—but one that resonates with the writer (a gentle soul may be ill-advised to attempt a horror script).
More avantgarde screenwriters and playwrights, of course, march to far different drumbeats, which precludes much in the way of commercial concerns. Among these writers are those who pioneer the dramatic arts of their generation. Ironically, in time, their vision is subject to becoming mainstream.