Alec Guinness did not receive an Oscar nomination for portraying Father Brown in The Detective.
The film itself lives and dies on the shoulders of Alec Guinness, and a bit on Peter Finch, as Father Brown. This is again Guinness, very notable within the time, going for a sort of chameleon style of performance. Not through overt makeup or anything, just through his performance. Now Guinness previously played a priest in Kind Hearts and Coronets, also directed by Robert Hamer, again in a bit of a disappearing act. Guinness though finds yet another unique characterization in his creation of Father Brown. Guinness begins with his alteration of his accent just ever so slightly, that just seems so fitting to a beloved sort of local priest. Guinness though continues in his physical manner that is all his own here, as even specific behavior, such as a moment of imagination, Guinness fashions something unique to his Father Brown. As typical for Guinness though he's both incredibly consistent in the mannerisms he grants to his character, and perhaps more importantly he makes this feel wholly natural to the role. It not only feels natural to the role, but it succeeds in amplifying it as you just feel you're with Father Brown after a few scenes with Guinness.
Guinness is especially important to making this character work as there are several elements that need to be made sense of. Guinness on the first aspect must be the sleuth of course for any good mystery, or crime film of this sort. Well, as he showed to an even further extent in Tinker Tailor Soldier Spy, Guinness is a master of the incisive stare. Guinness though does this in a rather fascinating way here as there is a softness within this in a strange way that seems just right for the character. Guinness though provides sort of that detectives spark of intuition in his work so well through he sheer energy he brings as Brown goes about trying to solve case. Of course none of this is at the expense of his portrayal of Brown as a priest either though. Guinness in the moments where Brown must fulfill his role in a sermon or in providing guidance to a parishioner, Guinness offers that grace and dignity within his performance which does not at all seem at odds with the rest of his performance. Guinness rather shows very directly though that Brown is one man, but even he is technically aware that he is a professional priest, but not quite a professional detective.
Now the priest being a detective may seem a touch far out, but Guinness even makes this absolutely work through his approach. The idea is quite mad after all as shown in the opening scene where he arrested after returning stolen goods for one of his flock, and Guinness plays into this so effectively. Guinness does this by bringing a bit of madness into his performance, now not that he is psychotic or anything like that but Guinness brings the right eccentric kookiness to the role. Guinness makes this an essential motivation for his performance actually. In that he makes that sort of insanity a part of who Brown is and this is needed to partially make sense of the man. He is a bit off his rocker and Guinness uses this without compromising the character in the slightest. Again Guinness uses this to amplify him so well by creating an understanding in his viewpoint even if it is more than a little atypical to say the least. Guinness makes the whole central concept work by playing into that it is somewhat ridiculous but never making fun of it either. He instead makes Brown a man capable of being what he is which includes being not quite the most normal priest.
A major difference between Brown and most amateur sleuths though is his intentions. He intends to stop criminals, but that is not his primary intention as he first and foremost desires to save their souls. The central plot follows Brown as he wishes to stop the thief Flambeau (Finch) by not having him arrested, rather by trying to convince him to move towards a more righteous path. This is an element that Guinness properly offers a great deal gravity to and properly takes the most seriously. Guinness as he describes his purpose offers the strongest passion within his work, by creating such an honesty to Brown's words to help who he sees as a man lost. I would be remiss though not to also mention Finch who also gives a very effective performance and makes for a great scene partner with Guinness here. In that both together help to grant a certain reality to the idea of stopping the thief by convincing him that his life path is wrong, rather than sending him to prison. The two are great because both actually provide such an earnest conviction to their points of view. Finch bringing a proper cynicism which is so well counted by Guinness who manages to deliver Brown's speeches with grounded idealism. The certainty that Guinness brings not only is powerful but almost rather profound. Guinness makes an especially strong impact in their final confrontation as he delicately explains the way Flambeau has set up his own private prison. Guinness refuses any smugness yet rather brings this tender concern and warmth, showing Brown's attack on the man's life as an act of unconditional love.
Of course as much as Guinness excels in every facet of the character and making Father Brown a cohesive whole, it must also be said that this is just an incredibly entertaining performance to watch. Guinness is prime Guinness here which means it is just fun to watch him work, and he enlivens every scene with his mere presence. Whether it is trying out his various wrestling holds, or coming up with his own new plot to catch Flambeau Guinness is effortlessly engaging. Guinness is just an enjoyable detective and tha kind of would have been enough for this to be a very good performance, but he does go more than a few steps beyond that. As this is a brilliant realization of what is really a tricky character. It would have been easy to make him feel too sanctimonious, too bland or even annoying. Guinness not only avoids any such pitfalls he instead merges the threads of the character to make a single compelling figure for us to follow through his story. He is indeed the efficient detective, he is indeed the devoted priest, he even is just barmy enough to connect those two things, yet only makes the task of the man an absolutely endearing ideal. This is wonderful work from the always exceptional Guinness, and it's a bit of a shame that he never once returned to the role.