Given its title and its makers, the Coen brothers’ new movie might conceivably be a 105-minute proctology joke, but actually it’s an incisive, melancholy drama, with some funny bits, about the burgeoning folk-music scene in early-1960s New York. (The very fine 1963 album Inside Dave Van Ronk is a clear inspiration here.) Oscar Issac plays the eponymous anti-hero, a true but willful talent who won’t compromise, and this wintry week-in-the-life tracks him up and down Manhattan and halfway across the country as he struggles toward a personally acceptable version of success. It’s a cautionary tale, but not condescending, and all the more moving for it; the Coens can be so good when they just get over themselves. The music is consistently fine, the design details expertly observed, and Isaac seems exquisitely situated between standout supporting performances by Carey Mulligan as a fellow scenester and combative lover, John Goodman as a bloviating jazzman, and F. Murray Abraham as a Chicago impresario. Mulligan’s character in particular deserves a fuller arc than she gets, and some scenes’ intentions seem fuzzy, but the filmmakers get away with that because it’s all done with such extraordinary, experience-abetted confidence — and sincerity, which is nice, and a relief, to see from them.