Fellow blogger Martin Edwards has a brand new article on "The Detective in British Crime Fiction" which, as everything he writes, is well-worth a read.
"Originally, Martin writes, detectives tended to be memorable for their eccentricities; now the emphasis is on in-depth characterisation."
This is way too sharply drawn a divide in my opinion.
1°) Early detectives were not defined only by their eccentricities - actually, most of them had none to speak of: neither Thorndyke or Father Brown or Gabriel Hanaud can be called eccentrics; what set them apart from the rest of mankind is their deductive abilities. Only when the Golden Age began did the Sherlock Holmes model of the detective as a quirky genius become prevalent.
2°) I don't believe things have changed as much as Martin claims. Sure, we know more about the private, inner lives of modern detectives than about those of, say, Dr. Fell or S.F.X. Van Dusen. They appear to be three-dimensional characters insofar as they are allowed to exist on a greater scale - they are not meant just to investigate a case, solve it and go back in the box until next time - but they too have eccentricities of their own, though of a different nature. One of the most overlooked features of the genre is its almost complete unability to deal with ordinary people. Mystery, even at its most "realistic", needs protagonists bigger than life in one way or another. Contemporary detectives with their complicated backgrounds, difficult lives and sometimes outlandish personalities, are as statistically improbable as their elders and betters. There are as few Dalglieshes and Rebuses in real life as there are Holmeses or Poirots.
"Anthony Berkeley's vain, erratic yet irrepressible writer-sleuth Roger Sheringham, Nicholas Blake's Nigel Strangeways, and Edmund Crispin's breezy don Gervase Fen are notable for the ingenuity which they bring to solving a string of elaborately contrived murders."
I am not a maven on either Blake or Crispin, so I won't comment as far as they're concerned, but Martin is some reductive with regards to Sheringham whose personality seems to me just as notable, and maybe even more so, than his ingenuity. Berkeley, like Sayers, quickly became frustrated with what he perceived as the limitations of the detective novel but, unlike Sayers, chose to go subversive rather than attempt to "transcend" the genre. Sheringham thus is both an embodiment and a scathing satire of the Golden Age detective, as evidenced by the wonderful Jumping Jenny where he gets everything wrong from the start and, thinking the victim had what she deserved, spends the whole book trying to clear the man he wrongfully identified as the murderer. That sets him apart from most other Golden Age detectives whose creators "played straight".