“A Real Pain” is a fluidly blended amalgam of pleasing, approachable subgenres, including an odd-couple buddy flick, a consciousness-raising road movie and a charged family melodrama. These story forms add to the overall sense of familiarity as does the focus on David and Benji, who emerge more through the complexities of their relationship than through individual quirks of personality. We are who we are, Eisenberg says, because of the people in our lives, a truism that becomes more stark and affecting as his characters travel through a country haunted by Jewish ghosts.