It's a good rule. Like any rule, it can be broken, but most of the time, it is good to keep in mind, especially when it comes to character development.
I often give setting a pass (though not always) because a quick tell--It was a deserted farmstead--is infinitely preferable to six pages of detailed description. Hey, let's move on to the characters!
But with character development, it is truly not a good idea to tell readers what they should think about characters, rather than letting (1) the characters' behavior tell readers; (2) other characters in the story tell readers.
I read a great deal of M/M romance, and a large percentage of it is very good. But lately, even from writers I've trusted, I've been encountering "homophobic" and "bigoted" and so on and so forth scattered amongst the pages without context. The writers seem to think that I'll be so overwhelmed by the terminology, I won't notice the lack of "show."
I'm not.
Especially since, in some cases, I have zero idea why a particular character was labeled in this way. There seems to be some secret code that indicates who the bad guy is and who the good guy is--not, ya know, actual behavior or actual dialog.
It's lazy writing.
It also makes my skin crawl since it is like being back in high school where violation of a clique's "code" results in nasty comments.
And, by the way, nasty comments due to clique violation is exactly what it is.
Bullying is bullying, even when dressed up in critical theory and used by people spouting appropriate doctrine. And yes, I apply that rule-of-thumb to conservative religious people as much as I do to progressive folks. Calling people names is rude. When the point is not to be rude (which has its place) but to tell the reader how to think, it's really rude.
Writers who try to "keep up with the Joneses" may find themselves further behind the curve than if they just ignored the Joneses in the first place.
