**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][1]


  [1]: https://www.merriam-webster.com/dictionary/adverb?utm_campaign=sd&utm_medium=serp&utm_source=jsonld