I saw "Papillon" on its initial release in 1973 and I found on rewatching recently that I had forgotten most of it. I am not sure what that says about the film, but after decades I was only left with the general impression that a prisoner's life on Devil's Island was horrible.
The story is true to some degree. Whatever its authenticity, the film is impressive in its depiction of a horrible reality--for Devil's Island did exist. And it is a filmic testament to the human spirit and its desire for freedom. Much of the credit goes to Steve McQueen and Dustin Hoffman, who played the two leads. If one is measuring McQueen's acting ability, it might be wise to start with his portrayal of the titular Papillon. Hoffman, on the other hand, has many other tours de force to his credit.
Among the canon of prison films, "Papillon" is certainly first rate. Personally, I prefer "Cool Hand Luke" or "Bridge on the River Kwai", but "Papillon" presents a stark captivity with little dialogue, relying on the camera's close investigation of the minutiae of daily existence as reflected in the prisoners' faces and their bearing. Did McQueen or Hoffman deserve the Oscar for Best Actor? They might have split the vote if both were nominated, but each probably deserved a nomination.
