Playwright Stephen Karam talks about his dark comedy “Sons of the Prophet,” with stars Joanna Gleason and Santino Fontana. It’s about a man who suffers from chronic pain and carries the fate of his reeling family on his shoulders. His health, sanity, and insurance premium are on the line. It’s been called the funniest play about human suffering you’re likely to see. “Sons of the Prophet” is playing at the Laura Pels Theatre through January 1, 2012.