“This is the story of one week in my life. I was seventeen. It was the week I slept in Orson Welles’s pajamas. It was the week I fell in love. And it was the week I changed my middle name – twice.” That is the opening line of a charming novel by Robert Kaplow about Welles’ famous Mercury Theater production of “Julius Ceasar,” which has now become a charming film from Richard Linklater (“School of Rock,” “Before Sunrise”), starring “High School Musical” heartthrob Zac Efron.

Welles is played by British theater actor Christian McKay, who starred as Welles in a play called “Rosebud” and perfectly captures the legend’s cadences and presence without making it an imitation. It is a true performance, and one that astutely conveys Welles’ galvanizing talent — and the infuriating single-mindedness that may be necessary to achieve his brilliant productions but never looks back at its shattering effect.

Efron plays Richard, a high school senior Welles impulsively brings on to play Lucius in the production that is about to open. Claire Danes is Sonja, Welles’ ambitious assistant. And the Mercury repertory company, many of whom would go on to become established theater and movie stars, are there for fans of “Citizen Kane” and the 1930’s to appreciate: Joseph Cotten (James Tupper), George Coulouris (Ben Chaplin), and John Houseman (Eddie Marsan). The tumult and brinksmanship that goes into any theatrical production are deftly presented, and as we see everything through the eyes of Richard, a bright, confident, dedicated, but inexperienced newcomer, we appreciate the brutal demands but also the passionate commitment, and the thrill, of presenting something that everyone knows will be an unforgettable experience for the performers and the audience.

Efron turns out to be a real star, with enormous screen charisma that works well for the character, making us understand why Welles and Sonja are drawn to him. But he turns out to be a real actor, too, very much part of an ensemble, with one of his most impressive achievements how effectively he blends in so seamlessly. Utterly effortless, whether talking to another teenager with artistic ambition (Zoe Kazan as aspiring writer Gretta) or asking an older woman for a date, Efron is always engaging.

We know from the beginning that Richard will be disappointed; that is inevitable in any coming of age story. But we are confident that he will also develop the perspective to make the most from what he has learned. The glimpses of the actual modern-dress production, gorgeously staged, resonate and inspire. We leave looking forward to seeing more from Welles, and from Efron, McKay, and Linklater as well.