Verbs that describe body positions:

"Mr. Dursley sat frozen in his armchair." in a book called Harry Potter and the Sorcerer's Stone.

to sit, stand, kneel, crawl, for example. These are all intransitive verbs.

They do not take objects but are often used with adverbs:

• The baby sat happily in his high chair.
• The man stood shakily on his injured legs.
• The boy crawled quickly across the floor.
• The priest knelt carefully at the altar.

frozen is usually an adjective: The frozen ground was very hard.

However, in "sit frozen", that describes how the person sat, just as in sit contentedly, sit stiffly, sit elegantly. It functions as an adverb. It describes the manner in which he sat.

Essential Meaning of adverb : a word that describes a verb, an adjective, another adverb, or a sentence and that is often used to show time, manner, place, or degree In "arrived early," "runs slowly," "stayed home," and "works hard" the words "early," "slowly," "home," and "hard" are adverbs.

Merriam Webster - Adverb

Verbs that describe body positions:

"Mr. Dursley sat frozen in his armchair." in a book called Harry Potter and the Sorcerer's Stone.

to sit, stand, kneel, crawl, for example. These are all intransitive verbs.

They do not take objects but are often used with adverbs:

• The baby sat happily in his high chair.
• The man stood shakily on his injured legs.
• The boy crawled quickly across the floor.
• The priest knelt carefully at the altar.

frozen is usually an adjective: The frozen ground was very hard.

However, in "sit frozen", that describes how the person sat, just as in sit contentedly, sit stiffly, sit elegantly. It functions as an adverb. It describes the manner in which he sat.

Essential Meaning of adverb : a word that describes a verb, an adjective, another adverb, or a sentence and that is often used to show time, manner, place, or degree In "arrived early," "runs slowly," "stayed home," and "works hard" the words "early," "slowly," "home," and "hard" are adverbs.

Merriam Webster - Adverb