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It's hard to know what exactly you are talking about without an example, but in general you can use a little "Addendum" in your answer to talk about off-topic stuff. Just as an example: if the OP asks for a way to "show don't tell" that characters X and Y like each other and your off-topic suggestion is not to name them X and Y, because those are the names of the protagonists of a well-known novel, which will likely invoke a certain feeling in the reader that the OP doesn't want to invoke you can structure your answer like:
To show their feelings for another you could use the fact that [stuff relating to the example posted by the OP - and the rest of your answer, probably going on for a few paragraphs]
As a little side-note:
I wouldn't name them X and Y. [Name of the other novel] has the same protagonists name and that's a novel about [controverse topic], which might not be the mental image you want to invoke in your readers. You may want to look at other names. For some inspiration look for example [internet link] or this other question on Writing.SE: [link to one of the "how to create memorable names" questions]
If you don't have an answer but still want to tell the OP something you think is important you can leave a comment. Comments aren't heavily policed on this site like on others because we generally don't have so many problems with them. Just make sure it doesn't become a long discussion and don't always use them to criticize off-topic stuff. Comments are still foremost to ask for clarification. See also Do we have an official policy on answers in comments?
Or leave a comment and invite them to the chat so you can talk more about the topic. Especially if you are interested in a discussion about the thing you are pointing out you might want to take it to chat, as off-topic discussions are perfect there. So a little comment like the following might be okay:
Hey there, interesting question! I don't have an answer for you, but I would be careful with those names. [Novel name] has the same and is a bit... weird. If you want to brainstorm a bit about names you can find me in The Overlook Hotel, our sites main chat room: [link to The Overlook Hotel]
Chat is public and everyone can read it, but only users with 20 reputation can talk there, so be aware of that when recommending the chat. But maybe you just want to leave a longer off-topic message than what would fit into a comment, so leaving something in the chat and linking to it could work out.