There are rules.
Sometimes there are rules that are there just because there have to be rules. The French know this. It is important to have rules, so they have rules for everything. On Paris Metro trains, there used to be a rule listing the order of priority of people to whom you should give-up your seat, because in France they know that there have to be rules. However everyone ignores the rules because France is an egalitarian society, and nobody has the right to impose rules on anyone else.
Some rules are there for good reasons. In dramatic writing, one of the most frequently quoted rules is “show, don’t tell”. The whole point of dramatising something is to show the story; if you want to tell it, you use some medium other than the stage. “Show, don’t tell” has been around a long time. Shakespeare knew the rule – and he broke it:
To be, or not to be…
You can argue that the whole of a soliloquy is about telling: it’s just the character talking to the audience. However Shakespeare’s soliloquies and the best monologues (which, by definition, do the same thing), use telling in order to show us something else: in this case, Hamlet’s state of mind.
Telling instead of showing is at its worst when characters give each other information that they already know. If the audience can hear that an exchange is there purely to give them information, then it’s bad writing. It’s bad because it’s distracting. It’s taking the audience away from their suspended disbelief and reminding them that they’re watching a play. This is at its most obvious in writing for the radio – when a character says something purely to tell the listener what is going on. I discussed this with Damian Trasler a while back and he gave the example: “The gun that I have in my hand is loaded.” You can hear the dialogue go “clunk”.
On the other hand, Douglas Adams deliberately bent that rule for comic effect – with more-or-less Damian’s example. I can’t remember whether or not it occurs in the books, but it’s in the seventh episode of the original Hitchhiker’s Guide to the Galaxy radio series:
Zaphod: Look, if it’ll help you to do what I tell you, baby, imagine that I’ve got a blaster-ray in my hand.
Captain: You have got a blaster ray in your hand.
Zaphod: So you shouldn’t have to tax your imagination too hard.
There are plenty of occasions when a writer needs to give information to the audience, but it needs to be done creatively, so that the audience hears the dialogue, not the plot.