The most effective low budget scripts are targeted for a niche audience. Maybe everyone will want to watch your film, but 'everyone' requires tons of marketing. More marketing equals more expense. So, focus.
Defining The Audience For Your Script
That audience should be clearly defined, maybe by age, gender, or habits. You don't have to get into the weeds on this. Define the audience for your script - first for yourself and stay in tune with what they want. The audience for any movie is intrinsically tied to its genre. By its very nature, horror movies generally are not created for a g-rated audience. Family audiences and young children are not the target for horror movies. Faith-based movies are typically written for family audiences based on Christian values and beliefs.
So much of audience depends on age. That tends to be true with thriller and action movies as well, skewing to a teen to adult audience. Horror, thriller and action movies contain things that may frighten young children, or are considered to be too intense. There is also an avid audience of adults (18 and up generally) that really want these films, and know the types of scenes in these movies and want something new. Something new often translates into something very intense. That typically puts these movies in the category of expecting an R Rating (from the MPAA). You cannot predict the MPAA rating of course, but you can assume the type of content that would be included in terms of sexuality, language, swearing, drug use, explicit violence, gore, adult situations and so forth.
Get as specific as possible to inform the story of your screenplay, then get going. Don't make a meal out of it.