There comes a point about halfway through "Building a Character" when Stanislavski's earnest young protagonist, Kostya, complains to his mentor, Torstov, that this batch of lessons all seem rather technical and not nearly as much fun as all the creativity and inspiration that was the foundation of the last batch of lessons (found in Stanislavski's first volume, "An Actor Prepares"). As a reader, you're bound to feel the same way.
Torstov's response is to tell Kostya (and presumable you, the reader) to stop whining and points out that the theater is not all getting in touch with your inner feelings and being artsy and creative. There's a lot of tedious and difficult work that lies in the preparation of your body and voice. This preparation is the canvas on which you can use your creativity and artsiness to paint successful characterizations. Without the strong foundation all this preparation creates, your acting will be a hollow imitation of real life, the worst sin imaginable by Stanislavksi.
As Kostya shuts his trap and does as he's told, you begin to understand that whether or not you can bear with the instruction offered in the pages of "Building a Character" will indicate clearly how serious you are about acting.
It is no great effort to be creative. It is something that inherently artistic people can do with almost no thought at all. But Stanislavski argues that concocting interesting ideas and clever interpretations can only take a performer so far. It is sharing those ideas with your audience in such a way as to make them believe the reality you are creating that is important, and without a finely tuned instrument (i.e. your body), your efforts will always fail.
To that end, the author spends most of these pages outlining in great detail strategies and exercises to develop the physicality of your body, the versatility of your voice and vocal patters, and your sense of timing and rhythm. And like Kostya, you will likely feel like skipping over some of the tedious instructions Torstov delivers. But if you make yourself read every word on every page, you'll find that every single thing Torstov says is indeed quite useful.
"Building a Character" is not as fun or inspiration a read as "An Actor Prepares", but it is certaingly every bit as useful.