When an author says to me:
"The main character in my next book is a Physicist at CERN; the problem is: I don't know anything about the physics that is done at CERN,"
You can imagine the alarm bells ring. A persistent member of the "Writing Advice Top 10" is Write about what you know. It is, this good advice notwithstanding, my observation that a writer with sufficient skill can convince his readers that a character is a genuine nuclear physicist even though the writer has even less knowledge than, for example, a press officer for British Nuclear Power.
How do you convince the reader? You only need go so far as to say that your character is a physicist and holds a senior position at CERN and other characters treat him as if this is so. The writer can spoof all kinds of skills and expertise in this way, because the reader is never going to challenge him on it, for purely practical reasons.
A writer can go so far as to invent a character who is himself a far more talented and celebrated author than his creator. How? By just saying so. No reader is going to ask to read the character's books. In the same way, the writer can invent a demon violinist or the greatest composer the world has ever seen. To convince the reader all he has to do is convey the experience of listening to the music to the reader, or (more detached still), describe the consequences of listening to it; crowds of adoring fans, squaddies and schoolteachers in tears, and whatnot.
I admit that the last couple of examples require a certain skill. But this is a skill in writing, a skill that I tend to assume any writer is pursuing.
The poet, however, is a trap.
It is the very nature of poetry that the experience of poetry can only be communicated by the poetry itself. Go on, find someone who has never heard the 18th Sonnet, and try to convey the feelings it evokes without using either any words or any images from it.
I think there are very few fictional poets for exactly this reason. You have to be an outstanding poet to write a poet character because there is neither any excuse for giving no examples, nor any way to convince the reader that the character is a poet other than by showing his work. Racine managed it. Hugo managed it*. It took giants of technique and inspiration to convince readers that their characters were passable poets. Perhaps the most internationally famous example is the character of Cyrano de Bergerac in the play by Rostand. Not that Rostand's work is extraordinary - but since it is written entirely in rhyming couplets, we can easily accept that both writer and character are competent at poetry.
It even takes a certain skill to write bad, mediocre or otherwise indifferent poetry. It did occur to me that you might have some other character remark:
"The say the fellow is the greatest poet of his generation but if he is he's damn discreet about it. I've yet to hear so much as a troche."
However I fear all this will do is whet the reader's appetite all the more. Even if the damn poet is the victim (i.e. already dead at the start), his Eratian credentials still have to be established somehow.
I suppose the canniest author might make a selection of contemporary anonymous poetry and claim it as the work of his fictional character. That's the kind of devious thinking that you need from an editor, in any case.
* At least, I'm pretty sure he did. But I can't find the reference and it's making me crazy. Please let me know if you know which character I'm thinking of. Please.