You're (the) one to talk

You're one to talk.

This expression usually means that the speaker thinks the listener shouldn't be talking in the given situation. So it's sarcasm.

Sometimes, you can make it more sarcastic by adding 'a fine':

You're a fine one to talk.

Could either be used literally to mean that the speaker thinks the listener should be talking in the given situation?

If not, how can you make it express the literal meaning? Perhaps by adding 'the'?

You're the one to talk.

Solution 1:

Could either be used literally to mean that the speaker thinks the listener should be talking in the given situation?

No. Fluent speakers will always take these expressions to be meant sarcastically, and because that's the case, they would avoid using such an expression when they want to be taken literally. Even in an unlikely situation such as Sven Yargs describes in a comment (where the person has been chosen to talk), a fluent speaker would avoid using the construction "you're [a fine] one to talk" because of the likely interpretation of sarcasm; at the very least, it would instead be phrased "you're the one to talk".

If not, how can you make it express the literal meaning? Perhaps by adding 'the'?

Using the definite article does makes a semantic difference here. Instead of the person belonging to that undefined conceptual group of people who should not be talking (because it would be hypocritical to do so), the definite article defines the person as specifically one to talk – i.e. it flags that a literal interpretation may be required. Nonetheless, we English speakers seem to revel in sarcasm and we're therefore constantly on the lookout for it, so a fluent speaker may well consider that the other person has simply mangled the idiom and actually is being sarcastic. The definite article, on its own, may not be enough.

In addition to using the definite article, the key to expressing this so that the literal meaning is conveyed without any doubt is to add qualifiers that unequivocally eliminate sarcasm as the intention. These would typically emphasise the context, or highlight the appropriateness or truth of the statement – and it would be common to use both, for the avoidance of doubt! For example:

• In this situation, you're genuinely the one to talk.
• You're absolutely the right one to talk.
• I honestly think you're the one to talk.

We also use vocal inflection to convey meaning. In the sarcastic expression we would typically put the stress on you (or you're), in effect emphasising the target of our sarcasm; when conveying the literal meaning we would shift that stress to the verb: "you are the one to talk".