In an interview, the Duffer Brothers opened up about the unexpected (and adult) changes that that Wolfhard brought to the Stranger Things production, following his experience working on IT.
The two brothers ultimately redirected their creativity towards an original series, as opposed to an adapted film, and thus bore Stranger Things – as well as their working relationship with Wolfhard.
They even reiterated that the Stephen King book on which IT is based was an inspiration for the Duffer Brothers when creating Stranger Things.
That’s the thing… after he [Finn Wolfhard] came back from shooting IT – he did it before we shot shot season two – he was f-bombing everywhere.