So what is subtext? well put simply it's the thing that isn't said by your characters, but rather implied, and in such a way that the audience understands what the character really means. Without it your characters will be flat, two-dimensional and uninteresting, with it they start to have depth and complexity and if it's done well it not only lifts your characters but it lifts your whole screenplay to a different level and raises your game as a writer.
So far so good and that's where Seger's book starts, defining subtext, but where most books stop, that's where this one really gets going, covering subtext in terms of not just dialogue, but emotions, sexuality, gestures and actions, conscious and unconscious thought, imagery, culture, genre and even sound. In particularly I enjoyed the section on gestures and actions. They are by far and away the more subtle forms of subtext but are really powerful tools when mastered because, I feel, they speak to us on a much more visceral level than words ever will. I have to say that I was blown away by a couple of the examples because I found myself thinking "now that's why I love that scene..." where previously I hadn't been able to articulate exactly what it was that I liked.
As you'd expect the book is nicely written, in a friendly and easy to absord style, with each form of subtext picked apart and explained in some detail. There are plenty of examples too, all the way from 1937's Snow White to 2010's The White Ribbon and they cover every genre from costume drama to zombie flicks so it caters for everyone's tastes; in fact a more comprehensive example set - in any book - would be hard to find.
It is a slim volume, only 150 or so pages, which is a shame because you don't want to stop reading once you get going but this is one of those 'quality over quantity' occasions because every page is full of insight with not a word wasted and it's definitely one of those texts you'll keep close to your writing desk to refer back to time and time again. If you're serious about improving your writing then this book is a must-have for your library.