Indefinite (and definite) articles are sometimes associated with a person's name. This answer by Jon Hanna is the best summary of the uses I have found. Also, another question addresses the issue that was not mentioned in the summary with the example sentence "when a defeated Napoleon rode off the battlefield and into exile", and it is followed by a couple of useful answers.

My question is related to the latter. Let me add three more examples to clarify the point.

In any of these cases, as well as the Napoleon-related example, the person(s) referred to is precisely specified in the text and only one in the world.

I have heard of a hypothesis (from a friend) people sometimes use an indefinite article before a particular person's name when it is accompanied with a preceding modifier, which tends to be (though not necessarily limited?) the present or past participle form of a verb; in other words, the modifier is sandwitched between the article and person's name. All the cases listed above fit the hypothesis and the modifiers are "startled", "elated", and "unsmiling" and "beaming". If this hypothesis is correct, there is no expression like "a John Smith delighted with blah-blah" (as long as the referred John Smith is already narrowed down to a particular person in the given context).

An answer to the aforementioned question explains

It's a literary/oratorial device. You certainly wouldn't often come acrosss it in normal speech - only somewhat flamboyant prose or speechifying.

Think of it as meaning that Napoleon is a man of many aspects, victorious on other occasions, but defeated this time. Using the article implies that this is only one of several possible Napoleons that have or will exist.

The first part fits my little Googling research; it seems this use of the indefinite article is more commonly seen in novels and fictions than other types of writings.

Applying the second part to my examples, the three example sentences are interpreted as
"Fallon/Storey/Francis/Trump is a person of many aspects, but in this case s/he is only one (=startled/elated/etc) of several possible Fallon/Storey/etc",
which seems fitting.

Now I am asking for further clarifications:

  1. What is the condition where this use of an indefinite article is accepted? Is the above-mentioned hypothesis (i.e., always(?) having a modifier in between) correct? Are standard adjectives like "happy", as opposed to verbs in conjugation, accepted for it?
  2. What is the nuance of the expression? Specifically, what is the difference from the phrase without an indefinite article? For example, how about "'blah-blah', muttered startled Fallon"? I think the definite article "the" would feel a little unnatural in this case, as explained in detail in the paragraph in the previous answer.
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    Possible duplicate of Why is it “a defeated Napoleon, not “the defeated Napoleon” who rode off the battlefield and into exile? There's also related Indefinite article and people's names, but I don't think that's quite so relevant to your specific question here. Oct 8 '18 at 12:27
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    Note that there's a difference between, say, There's a Mr Smith on the phone and After winning the office sweepstake, a delighted Mr Smith phoned his wife to tell her the good news (where there's always an adjective between the article and the proper noun in the second form). Oct 8 '18 at 12:33
  • @FumbleFingers I didn't know the question with regard to a Napoleon-related sentence. Thanks! I think my quesiton is broader, and so I leave it as it is. I interpret in your first example ("There's a Mr Smith"), the speaker does not identify exactly who the person called Mr Smith is, whereas in your second example the speaker knows him. My question is related to the latter case. Thanks for confirming there is always an adjective in the latter case. Oct 8 '18 at 15:57
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    Well, as you've probably noticed, I posted an answer to the "Napoleon" question myself - I knew I'd addressed this point before, but it was 7 years ago, and I'd forgotten that the specific context referred to Napoleon, so it took me a while to find it. But I respectfully suggest that adapting a sentence in my previous answer: Think of it as meaning that Trump [or whoever] is a man of many aspects, scowling [for example] on other occasions, but beaming this time should exactly address the aspect of the usage you're asking about here. Oct 8 '18 at 16:11
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    As to "How informal?", I'd say it's not really a "formal" usage (more "stylised, poetic"), but it's certainly not a construction people would normally use in relaxed conversation. Nobody would be likely to say, for example, I spoke to an inebriated Mr Smith in the pub last night (in a context where both speaker and audience know perfectly well which Mr Smith is being referred to). But the stylistic device is common enough in the world of newspapers and magazines. Oct 8 '18 at 16:38