SHOW DON'T TELL, GODDAMMIT. Apparently. That's the standard advice, and to be fair, it's a fairly good rule to live by. However, when you go to the cinema or turn on any TV comedy/drama, there's plenty of moments where it's very much a case of TELL DON'T SHOW. In some cases, it's bad exposition, in others, it's perfectly fine. But why is that?
Bad exposition is the script giving us information before we've had a chance to work out what's going on for ourselves. It spoon feeds us story and cuts off our engagement, sometimes only in a minor way but enough to momentarily throw us off our stride.
'Tell Don't Show' is perfectly fine when we're fully aware of the story's dramatic context. If characters start saying exactly what they're thinking or feeling, we're usually invested in what they want to say, and how they want to say it. This is where even the most blatant piece of exposition contains a subtext of everything that's happened up until this point. For example, someone blurting out 'You're my wife. I love you' could be heart wrenching and emotive rather than dull and bland. Take this classic example from EastEnders (direct link, go to 1:21 in embed below), written by TV legend Tony Jordan.
'Show Don't Tell' is generally the preferable way to go but sometimes you just want to hear a good old barney between characters, and have them flinging all sorts or on-the-nose barbs at each other. If you try to be too clever with 'show don't tell', you risk confusing the reader/audience.
Here's a scene I wrote for EastEnders in 2009, where Archie Mitchell gets his cancer diagnosis confirmed. I didn't use any dialogue, and am very proud of the scene (and the way the EastEnders' team embraced the idea), but on the night, I noticed on Twitter that some of the key audience were still confused ("does Archie have cancer or not?"). You can't win.