By Stephen Whitty:

Growing up, Mike Myers' god was Peter Sellers; Sellers' was Alec Guinness. This blackest of black comedies shows why, as Guinness—long before multiple roles became a commonplace gimmick—plays eight members of the shortlived D'Ascoyne family. Dennis Price is the black sheep heir who does most of them in; the incomparably plummy Joan Greenwood and endlessly classy Valerie Hobson co-star.

From Alec Guinness: A Celebration, by John Russell Taylor (London: Pavilion Books Limited, 1984):

In Kind Hearts and Coronets high comedy rules. Or at first glance it does. It is only when you think about it afterwards that the serious side becomes apparent. Black comedy you can certainly say it is, but the blackness of this comedy of murders is no flip gesture. When Louis Mazzini determines to murder his way through his mother's insufferable family, who has rejected her and him, to a title and a fortune, it is a revolutionary gesture, even though it is all done with a patrician elegance out of Wilde. And for this gesture to have any meaning, the d'Ascoynes have to be, even within the comic convention, believable, and to a greater or lesser extent, monstrous. Any easy gallery of picturesque disguises would not do, and, to his infinite credit, this is not what Guinness provides.

No one seems to remember now exactly how the notion of his playing the whole family came about. At first the intention was to find eight different actors with a sufficient resemblance to play the family convincingly. Then at some point Guinness was offered three of the roles. He, it seems, thought this was rather silly: if three, why not all? And, in the end, that's how it was. Michael Balcon, the producer-in-charge at Ealing films, gave the idea his blessing, though in some ways it went against the traditional team spirit of the group, for however far Ealing Films might aspire to be a communal effort, he could not fail to be aware from his earlier career that a star could draw audiences in a way nothing else could. And already he recognized in Guinness the potentiality of film stardom.

Kind Hearts and Coronets was in other respects rather out of the Ealing routine. The studio's reputation—for already 'Ealing comedy' was well on the way to becoming a brand image—was for small scale, cozy, defiantly British films: dramas and picturesque comedies which showed the British to themselves the way they wanted to be seen. It was largely coincidental when, as a result of films like Passport to Pimlico, Whiskey Galore and Kind Hearts and Coronets, they became truly popular in the States and elsewhere in the world, simply demonstrating that with international audiences the best bet is to remain firmly and unflinchingly national. But Kind Hearts and Coronets, though as unequivocally English as any of them, fitted neither into the stereotype of cosiness and fundamental amiability nor into that of the group effort. For technical reasons it was made outside the family circle at Pinewood, and it was very definitely the creation of one man, the writer-director Robert Hamer. And the tone of epigrammatic ruthlessness was in the tradition of Oscar Wilde but also that of such very British classics as Harry Ghram's Ruthless Rhymes for Heartless Homes.

Describing working with Guinness, Robert Hamer, who was to work with him three more times, gave the impression that he was some sort of Method actor, entering so completely into his character that he continued to carry about with him the aura off-screen. If he was a good-time Charlie everyone was part of the fun, but if he was a senior lord you wanted to call him 'Sir'. This may well be true, to the outside observer, but Guinness himself has always taken the opposite view, even to risking the charge of mimicking rather than acting: he believes in starting with the outside and working in. Consequently his makeup for each d'Ascoyne was a crucial part in the individualization of the character—along with his observation of how such a person would talk and move. Once that was right, the rest would follow naturally.
In Kind Hearts and Coronets it evidently did. When the film came out in mid-1949 there was no doubt about it: Guinness was a film star, though one would have been hard put to it to say what sort of star. Not that definitions matter when public response is so unequivocal.