I just came across something I'd written a while ago that contained the phrase "follows close behind", and my first thought was that it was incorrect and should be "follows closely behind", i.e. to use an adverb describing how something is following.
The Google Ngram for the two phrases seems to disagree with this instinct, though, with only slight differences between BE and AE, and the shorter phrase must've made it past my inner editor back when I first wrote it... maybe because "close" can also be used as an adverb?
So can both be used interchangeably? Is there a grammatical reason one should be considered "more correct" than the other, or is it just a case of one particular usage of a phrase having become more commonplace for some reason or other?