Acted is a verb which can take an adjective in what looks like a modifying position. Some verbs are like that; they are called copular (or copulative or copula) verbs. Let's consider the verb look, where using an adjective and an adverb produce different meanings.
For some meanings, you have to use nice:
John looked very nice in his new clothes.
*John looked very nicely in his new clothes.
For others, you have to use nicely.
John looked at me very nicely.
*John looked at me very nice.
In the first example, very nice is in some sense modifying John, while in the second, very nicely is modifying looked.
For the verb act, you can use either an adjective or an adverb:
John acted very strange.
John acted very strangely.
and the meaning isn't any different between these two sentences (or at least not much). I would say that in the first sentence, John is acting as if he was very strange, and in the second sentence, John's actions are very strange. In this case, the meanings end up being the same. But consider:
John acted very quiet.
John acted very quietly.
Now, these mean different things. In the first, John is simply being quiet. In the second, John is doing something and trying hard not to call attention to it.