Should it be “carefully” instead of “careful” in this sentence?

Question

He paused outside the doors, taking stock of his men, careful not to give any sign of his thoughts. (source)

My understanding is the phrase "careful not to give any sign of his thoughts" is an adverbial modifying the main clause. In that case, shouldn't it be led by the adverb "carefully" instead of the adjective "careful"? If I am in the wrong and the original sentence is correct, what kind of phrase is "careful not to give any sign of his thoughts"? What is its grammatical function? And why can "careful" as an adjective be used here?

Answer 1

Careful acts as an adjective there. It's an example of ellipsis, which is "the omission of a word or phrase necessary for a complete syntactical construction but not necessary for understanding." Think of this way, with the missing word supplied:

He paused outside the doors, taking stock of his men, [being] careful not to give any sign of his thoughts.