Suppose you’ve got a character who’s a poet. This is dangerous territory, because readers are wary of characters who are poets or playwrights or novelists or artistic sorts in general, because the readers have a well-founded suspicion that any artistic character is liable to be the author him-or-herself in a thin disguise . . . but let’s say that you’ve got good and sufficient reasons for doing it anyway.
And suppose, then, that your poet-character at some point has to write, and possibly even to recite (or sing, if this is a fantasy story and your poet-character is not just a poet, but a bard) a poem.
And suppose, further, that this poem is supposed to be a masterwork, something so profound and affecting that it moves the villain to mercy, or the populace to revolution, or the poet’s beloved to bestow love in return.
If that’s the case, then do not write that poem. Do not give your readers the chance to read it and say, “Huh. That poem isn’t really anything to write home about.” Because if that happens, your reader will not believe in the villain’s mercy, or the people’s revolution, or the beloved’s affection — all those things that were supposed to have been caused by this masterwork of poetry will fail when it falls short.
What to do instead: Don’t show the work of art. Instead, show the other characters reacting to it. Show the villain weeping, the crowd picking up paving stones and chair legs and broken bits of pipe, the beloved person meeting the poet’s eyes and smiling at last . . . that sort of thing. The readers will believe in your great fictional work of art, because they will have seen your other characters behaving as though they were in the presence of greatness.
(Yes. I know that Shakespeare actually did a Great and Moving Oration live on-stage in Julius Caesar, but he was Shakespeare and the rest of us aren’t. And Tolkien crammed The Lord of the Rings chock-full of his own poetry, but point one, while he wasn’t William Shakespeare he was a competent versifier in his own right, and point two, his characters never asserted that their poems and songs were great works of art.)
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One of my favorite examples of how to do this right is The West Wing. There are several episodes that revolve around writing speeches, and where the payoff is that the speech was so awesome that people were cheering from their socks (or not).
The audience hears, at most, a couple of lines of these speeches, along with the attendant reaction from the adoring crowd. (“This is a time for American heroes, and we reach for the stars.” *crowd goes wild*). Now obviously putting in the whole speech wouldn’t work from a time and pacing perspective. But it also really helps enforce the idea that Sam Seaborn and Toby Ziegler are very good writers. Martin Sheen could make the phonebook sound good if he tried, but as we’ve seen from The Newsroom, Aaron Sorkin can only get away with so much ventriloquism before the illusion begins to unravel. The less we hear of his Moving Oration, the more believably moving it is.
Joss Whedon had the same problem with Buffy’s motivational speeches in Season Seven of BtVS; he solved it by making her lack of skill at giving those speeches into a running gag of sorts. (The “if you can’t hide it, hang a lampshade on it” solution to a story problem.)