Though he’s not the actor on a boat with a tiger, in many ways Khan has the most difficult part of “Life of Pi.” Playing the adult Pi, who is seen in flashbacks played by other actors but most notably Suraj Sharma, Khan is ostensibly the narrator of the film, recounting for writer Rafe Spall the 200-plus days he spent lost at sea. But in his storytelling, Khan manages to convey so much loss, joy, and tragedy that you feel his presence, even when he’s not seen.