An Actor Prepares is the diary of a fictional student named Kostya during his first year of training in Stanislavski's system. Kostya and his fellow students have little to no experience in acting. As they go through the class, Tortsov, their teacher and theatre director addresses the many assumptions they have formed that do not coincide with the 'system'. Stanislavski relates his message with examples. He argues that his system is not a particular method, but a systematic analysis of the 'natural' order of theatrical truth.[citation needed]

The system that he describes is a means both of mastering the craft of acting and of stimulating the actor's individual creativeness and imagination. It has influenced the majority of performances we see on the stage or screen.

The book is autobiographical and deals with many different areas of acting skills, including action, imagination, concentration of attention, relaxation of muscles, units and objectives, faith and a sense of truth, emotion memory, communion, adaptation, inner motive forces, the unbroken line, the inner creative state, the super-objective and the subconscious mind. Tortsov, the Director, explains all these art forms in great detail, and thereby transforms An Actor Prepares into a type of textbook.

The book begins when Kostya and his fellow students are waiting for their first lesson with the Director. They are excited and nervous at the prospect of meeting, and are surprised when he tells them that their first exercise is to put on a few scenes from a play. Kostya and two of his friends perform scenes from Othello, with Kostya taking the leading role. Afterwards the Director tells them their mistakes.

At the end of the book, the students recall their first exercise: Sitting in a chair in a way that interests the audience, and searching for a brooch convincingly.