How do you follow a film like Shoah? The nine-hour Holocaust documentary, released in 1985 after 11 years of work and 350 hours of interviews — with survivors and perpetrators, saviours and collaborators, historians and bystanders — is considered one of the greatest films ever made. For decades, director Claude Lanzmann kept returning to the subject, raking over the same material, finding it impossible, maybe indecent, to move on.