"She looked at him, fierce."

All of these suggestions are good.

My first thought was: "she looked at him, fierce" like a finger-snapping, neck-rolling diva-kind of fierce. If you're going for urban slang, it works. If she's just pissed though, fiercely works best.

I'd agree that 'fierce' could work if it's dialogue from a region where they talk like that but if it's neutral English narrative then the adverb is needed for grammatical correctness (and irrespective of whether there might be a better one).

Consider this from, eg, Devon or Cornwall in England -

"She looked at me, all fierce like. And oi be thinkin, now then Jonno me lad, why'd you 'ave to open that big trap o' yours?"

Cheers,
APC

Mr. Mask said:

... I was wondering if, "She looked at him, fierce.", would be acceptable in fiction writing.

I don't think it is correct grammatically. I've seen some authors skirt the line of grammar...

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I think sometimes skirting that line is a good thing. Keeps the writing vibrant, surprising (Hey, look - (s)he used an adjective to modify a verb! Did you see that? And the world didn't end).

But I also think the writer needs to know which sentences are worth fighting the grammar battle. I don't see that your example qualifies.

InspectorFarquar said:

I think sometimes skirting that line is a good thing. Keeps the writing vibrant, surprising (Hey, look - (s)he used an adjective to modify a verb!

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That doesn't seem to be what's happening here, though.

As others have said, if the modifier is meant to apply to "looked," using the adjective form is incorrect and confusing.

But I believe the construction is more akin to that of "He stopped in his tracks, stunned." In both cases, the final word, set off by a comma, is an adjective modifying the subject of the sentence. No one would lobby to change "stunned" to "stunnedly"; it's only because "fierce" has an -ly adverb form that some people have gotten misdirected.

So I don't see any line-skirting involved in this example. But the OP has never clarified his intent, so we're all just guessing.

What about being dramatic and making it short and brusque - "She looked at him. (Full stop) Fierce. (One stark word.)
One of these days I am going to write a learned paper or perhaps a profound novel entitled "The Death of the Adverb."

maryland said:

One of these days I am going to write a learned paper or perhaps a profound novel entitled "The Death of the Adverb."

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You could call it "We Don't Need Adverbs," but someone will point out that not is an adverb.

guttersquid said:

You could call it "We Don't Need Adverbs," but someone will point out that not is an adverb.

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That reminds me of a German book called "Das Dativ ist dem Genitiv sein Tod".

Got a few chuckles out of me, at least.