I saw JTTCOTE today for about the 150th time.... I think Pat Boone did an okay job in this role, arguably the best of his career -- granted, that's not saying much as most of his other films were fairly poor. Here, he's pretty effective, certainly overdoing it a bit in some scenes to be sure -- but acceptable, as a previous poster aptly put it.
James Mason was one of the finest, and underappreciated, actors of his era and I don't agree that he or anyone else in the cast "hammed it up". In terms of his performance, Mason was by far the best one in the film -- though it seems clear he was having some fun with the part, obviously not taking it quite as seriously as, say, A STAR IS BORN. But excellent, as always. Even the film's non-professional, the late Peter Ronson, was quite good and believable in his key role as Hans -- better than Boone, I think.
he has a very french insouciant vibe going.