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.)