Screenwriter Robert Bolt circles the mystery of Lawrence without ever diving into his center, and David Lean uses the young and beautiful O'Toole as a living sculptural object, fetishizing his blue eyes, blond hair and pale skin (bronzed by the sun as the tale goes on). Such a controlled and exact film might have been a success anyway with a different actor in the lead, as long as he was attractive enough to be a focal point for the spectacle (Marlon Brando and Albert Finney were considered).
But O'Toole humanized the hero in ways that seem miraculous when you consider the caginess of the story that surrounds him; he does it, I think, by making Lawrence's conflicted and volatile emotions transparent even when the film keeps his motives mysterious. You never think, "This movie is a mess" or "This character makes no sense" because O'Toole makes sure you see the character as a paradoxically ordinary extraordinary man, cocksure and idealistic and naive, inclined to get drunk on his own power and success, but always thinking, always questing, always figuring things out.