Though it originally belonged to an actor who was a decade older and a foot smaller, the role of Arthur Bach is a custom fit for Russell Brand. The comedian steps into the late Dudley Moore’s shoes as a drunken, irresponsible heir to a family fortune who’s leveraged into a loveless marriage just as genuine romance and maturity are budding. Where Moore emphasized Arthur’s alcoholism in the 1981 movie, Brand doubles down on his the character’s infantilism, dressing up in superhero costumes and giving grief to his nanny Hobson (Helen Mirren in a gender-switched role originated by John Gielgud). The funnyman generates sweet chemistry with Mirren and with the always disarming Greta Gerwig as his true love, while oozing contempt at Jennifer Garner as his bitchy fiancee, but the best moments come from Brand’s stream-of-consciousness ad-libs. The new Arthur is a worthy diversion, a slightly above average romantic comedy that depends heavily on its lead performance---which is to say it’s a lot like the old Arthur.