I was interested by the comparison you made with Thomas Allen...
... there's nothing "new" there, but it's what is conventionally known as the "Stanislavsky Method". Stanislavsky established what he called the "shkola perejivaniya" ("the school of living-through it", sometimes wrong translated as "school of suffering")... the idea being that instead of just doing what's conventionally called "business"*, the actor retreats into the psyche of the character he is playing. This is by no means an easy thing, and it involves some extensive self-hypnosis.
However, the point I am making here is that any work on this level needs extensive ensemble rehearsal with the other performers in the cast. Stanislavsky himself suggested this should be anywhere from six months to two years! Elia Kazan, the film director who embraced Stanislavsky's methods still suggested three months work on building the characters before rehearsals started.
But opera productions these days are often done on four weeks rehearsal... in fact I've been given as little as two. What is heartbreaking under such conditions is when on Day 01, you get a message saying that "Mr Major-Operastar is unavailable for the first week of rehearsals, but has sent his understudy to learn the moves for him". (You'll note that the production concept has now been reduced to the words "the moves", ie when to go on and off, and from which exit). It's not only common practice for major stars to swan in 2-3 days before the show (expecting you to then abandon your schedule of work or personal plans to walk them through it a couple of times), but opera-house managements expect it and plan for it. There is even a living to be made as someone's understudy - for example I know a guy whose entire living is attending rehearsal schedules for a famous baritone.
Under these circs, it's not at all surprising that some singers seem to grin their way inanely through every role they do, whereas others actually take the whole range of their character's emotions onto their shoulders. It's a question of professionalism versus money-making.
There are also performers who give the same performance in every production, no matter where it is. People can get tripped-up like this... a well-known baritone was contracted to do some of the original run of Jonathan Miller's RIGOLETTO at ENO. Failing to bother to come to many rehearsals, as he was fitted for his barman's costume for Act I he was heard muttering "'Ang on! Where're me bloody bells?"
* this is ruthlessly parodied in the "rough acting" books, where an actor required to play an old man wheels out "No 3 plus No 11 - stumbles along, cups hand to ear to indicate hard-of-hearing"
