Hmmm....what expert wrote “act defensive” - not English - act is a verb, therefore the word that modifies it must be an adverb - one cannot act strange, defensive, courageous or decisive - it’ s act strangely, defensively, courageously, decisively.
This is one of the confusing areas of English grammar and usage. Act is a verb but sometimes it is a linking verb, which connects a subject to a subject complement. The regular verb is modified by an adverb, but a linking verb links a verb to a subject complement, which is an adjective.
As a regular verb: “Clint Eastwood acted superbly in his newest film role.” As a linking verb: “The bully acted mean.”
A man can stand independently (regular verb) or he can stand strong (linking verb). A woman can smell professionally in a perfume factory (regular verb), or she can smell good (linking verb). The crowd appeared suddenly (regular verb). The crowd appeared happy (linking verb).
Some linking verbs are: smell, look, taste, feel, sound (related to the senses); and become, grow, be, seem, sit, stand (related to existence).