He can — and this is rare — act mind, and may be the only actor alive who could play a genius convincingly: Donne, for instance, Milton, Pope, or even Shakespeare…would be comfortably within his grasp. But he is not, and never will be a star, in the sense that Coward and Olivier are stars. Olivier, one might say, ransacks the vaults of a part with blowlamp, crowbar, and gun-powder; Guinness is the nocturnal burglar, the humble Houdini who knows the combination. He does everything by stealth. Whatever he may do in the future, eh will leave no theatrical descendants, as Gielgud will. He has illumined many a hitherto blind alley of subtlety, but blazed no trails. Irving, we read, was rapt, too: but it was a weird, thunderous raptness that shook its fist at the gods. Guinness waves away awe with a witty fingertip and deflects the impending holocaust with a shrug. His stage presence is quite without amplitude, and his face, bereft of its virtuosity of make-up, is a signless zero. His special gift is to imply the presence of little fixed ideas, gambolling about behind the deferential mask of normality. The characters he plays are injected hypodermically, not tattooed all over him; the latter is the star’s way and Guinness shrinks from it. Like Buckingham in Richard III he is “deep-resolving, witty”; the clay image on whom the witches work. An innocence, as of the womb, makes his face placid even when he plays murderers.
Whether he likes it or not (and I suspect he does), his true métier will continue to be eccentrics — men reserved, blinkered, shut off from their fellows, and obsessed. Within such minority men there is a hidden glee, an inward fanatical glow; and in their souls Guinness is at ease.
That is from Kenneth Tynan, Profiles, which is in fact a remarkable and remarkably good book.