As a child, Bradley Cooper would mime conducting an orchestra, and he asked for a baton from Santa. Decades later, as a filmmaker, he fulfilled his childhood dreams in the acclaimed new film “Maestro.” Cooper co-wrote and directed the movie, and co-stars as Leonard Bernstein, perhaps the greatest American conductor ever. In a pivotal scene, Cooper conducts the famous London Symphony Orchestra with a full chorus, in real time, through a performance of Mahler, which Cooper calls the “scariest thing I’ve ever done.” But the movie focusses less on Bernstein’s well-documented musical triumphs than on his extremely complicated personal life and marriage—as a proudly nonmonogamous bisexual—to the actress Felicia Montealegre, who is played in the film by Carey Mulligan. “I had no desire to make a biopic,” Cooper tells David Remnick, especially of a man whose life is so well documented. Despite his proven track record as a box-office draw and critical success, Cooper found himself on the receiving end of noes  from major studios when he shopped “Maestro” around. “It makes sense what they [said],” Cooper concedes: “ ‘It’s a huge budget. It’s a subject matter that no one will be interested in. We just can’t justify it.’ ” With rave reviews and a holiday release setting his film up for a likely awards-season run, Cooper should feel vindicated. “This movie… I made absolutely fearlessly,” Cooper says. “And I knew I had to because that’s a huge element in Bernstein’s music. It is fearless.”