The below quote is an example that Longman has provided to illustrate the usage of the adjective "incredulous". I believe in this example, "incredulous" in used as an adverb. Please help me understand how it can possibly be acceptable.
"You sold the car?" she asked, incredulous.
All I can guess is that the original sentence was something like "she asked, with an incredulous manner" before it underwent a series of mysterious ellipses!
This construction is called an ‘appositive adjective’ and is not uncommon in written English, although rare in spoken English. Multiple adjectives (with conjunctions) may be included:
She stared, disbelieving and dismayed. "You sold the car?" she said.
When the appositive is set at the end, as here, it gives an effect of being added, as supplemental information—which tends to highlight it.
It may also appear before a clause, sort of like a stage direction for understanding what is to follow.
Riven by a multitude of competing cultures, burdened with outdated institutions, the Habsburg empire lumbered heavily into the 20th century.
1. "You sold the car?" she asked, incredulous.
This is correct usage.
In this question, it is the woman herself that is incredulous (unable or unwilling to accept what is offered). It is used as an adjective here. The emphasis here is that the woman who asked the question is experiencing a state of disbelief.
But what about...
2. "You sold the car?" she asked incredulously.
In this case, it is used as an adverb. The focus is now more on what she is asking, rather than what she is feeling. Her entire tone, the way she is speaking, and her inflection in this case would be one of disbelief, possibly anger.
In the end, they both mean nearly the same thing. The difference between the two is subtle, with #1 her tone or voice may not betray what she is truly feeling inside. With #2, it would be very obvious to a listener that she was, indeed, incredulous.
