Hmmm, wish I knew what I had said in that master class ...
To answer Hex's question, older people tend to be set in their ways, so I'd say the older a bad, bad character is the less likely they are to change (not that they couldn't change, but that they are far less likely) -- except at that mid-life crisis stage, but that pertains to my next point, so I'll put that to one side for the moment. In fantasy and science fiction you often have characters who are very old indeed, and although they would have had plenty of life experiences to change them along the way, what they have learned (or think they have learned) from those life experiences would be very hard to shake. They are highly unlikely to go through any convincing changes, especially given the time frame of most books.
If the changes are too convenient for the protagonist, readers will be even more aware of how unlikely it is. For example: a villain who has been steeped in evil for decades or even centuries suddenly turns around at a crucial moment and says to the hero, "By your courage and goodness you have made me see the error of my ways! Nevermore shall I do such evil deeds. Now let me reward you with rich gifts and extraordinary favors, and rescue you from your dire straits." If something like that happens, readers will feel the writer has insulted their intelligence. Young readers may want to vomit. (But if, as in a manuscript I happen to be currently reading, an ambiguous character gives aid for ambiguous reasons, that is quite a different matter, and can be thoroughly convincing. But it would not have been convincing if the villain had done so.)
My second point is that people in general who are satisfied with their situation in life are far less likely to grow or change. A midlife crisis tends to highlight areas where a person is dissatisfied, or the things they feel they were deprived of earlier in their lives while they were doing what was expected of them or what they needed to do to get ahead, and a realization that after all none of it was as fulfilling as they had hoped. The sort of detective TJ mentions, who has become numb to the ugliness around him, has changed, not by growing, but by diminishing -- however convenient to his job, he has lost something valuable, the loss of which is certain to have an effect on other parts of his life -- which is also a character arc.
But if things are just as they want them, if they feel they have done all that they wanted to do, or at least that the lives they are living hold out endless opportunities ahead, even a person during those midlife years has no reason to want to change (short of divine intervention, a road to Damascus moment, but these are few and far between). A character who is immensely powerful one way or another (as so many bad, bad characters in SFF are) -- whether that power be magical, political, financial, or something else -- and who basks in the love, admiration, or fear they inspire, who adores all the advantages that power brings, would have no desire to change. They wouldn't even want to change their thinking, not very much, since what they think and believe would undoubtedly provide the justification for their nefarious deeds and iron grip on power. If robbed of their power and forced to go through a series of enlightening experiences, to learn how ordinary people live and struggle, etc. etc. they could be humbled and learn the error of their ways. Or at least ... a skillful writer might convince us that it could happen. But more likely they'll rage against the changes, since they had so thoroughly convinced themselves that they had a right to all the power and glory, the riches and comforts, they had before.
But young people do change in all sorts of way, especially when they are going through the kind of experiences that shake up their lives, where those experiences tend to be of an eye-opening kind. Even when none of that happens, unless they have been pretty well brain-washed by their elders, they change as a way of asserting their autonomy, of establishing themselves as unique individuals, rather than as extensions of their parents or their families. So it only makes sense that if you have young characters with aspirations, who don't lead ordinary, placid lives -- and ordinary placid lives don't make for very exciting reading -- that they would have character arcs, either to change directions in some way, to grow (or sometimes to diminish), or at least to become much more of whatever they were at the beginning.
And finally, people who don't change, who just go ahead doing the same things, thinking the same thoughts, are boring. Readers will forgive a lot -- given the right justifications, or the right amount of remorse afterwards, they will forgive some pretty heinous deeds (the slaughter of multitudes is nothing to them, given the right circumstances) -- but characters who bore readers, because they are too dull and do the same things over and over, or because they are too perfect and have no need to change, have committed the one unforgivable crime. (Note that it doesn't matter if the other characters find them tedious, if they amuse readers.) That's why readers often like the villains better than the heroes. The villains may not change, but at least they aren't boring.
So for the book springs referred to, where I asked for character arcs (for primary characters, and for important secondary characters) they were all young people. Some were part of and/or witnesses to extraordinary events, some had extraordinary intelligence and/or talents, some were training for extraordinary positions of responsibility. And for some of them more than one of these things was true. It seemed impossible to me, especially given the span of years the book covered, that these young people would not have character arcs. Without them, the book would not be convincing. And to keep readers from growing bored with them, to keep readers from wanting to take a nap during any scenes where they were the point of view characters, character arcs were also important.
