John Huston's MOBY DICK is a superior film in many regards. [I think Eric Kuersten, though to be commended for bringing the picture to our attention again, by any means necessary, is just being another blogger here, indulging a desperate need for a new hook.] I always thought Gregory Peck's performance as Captain Ahab was the epic's release version's only weakness. However, the restored version on laserdisc and now DVD, with Ossie Morris's unique experimental photographic process, gives us an Ahab often shown in subdued light and shadow, making Peck's performance much more effective than in bright hued, sunny technicolor.
In a recent comment to another thread, I noted the quite reckless challenges Huston gave to his actors, sometimes endangering and exhausting the likes of Clark Gable, or causing them lasting injury, as in the case of Audrey Hepburn. I did not bring up Gregory Peck in that regard, but in his carefully self-censored autobiography, An Open Book, Huston can hardly contain his pleasure in relating his staging of Captain Ahab's death (illustrated in the grabs presented in Mr. Kuersten's Blog). Shot within a big studio tank in England, his scheduling of the sequence was saved for last, so as Huston described it, should Peck have been killed or injured, the film would have been complete in any case.
Your contrast of Orson Welles as the corruptly charming Harry Lime, THE THIRD MAN, with his masterful Father Mapple in MOBY DICK (another of his best on screen performances) an unlucky seven years later may have its counterpart, Harvey, in what a greater passage of time had done to Gregory Peck in Franc Roddam's decent 1998 TV remake of MOBY DICK. The former Ahab is born again as Father Mapple, his final on screen performance, a quite gaunt, a bit doddering figure, like Welles if he had gone on an epic crash diet at the end of his life which left him enervated. I found Peck's performance the least successful in the picture.