“Stranger Things” has brought Hopper back for Season 4, and nobody’s gladder than David Harbour, who wants to peel back the layers of the character and reveal “the hero that you've always wanted him to be” (as long as he survives this season, that is). In this episode of “The Envelope,” Harbour shares his thoughts on portraying a cop in the midst of national police brutality conversations, the upsides of finding fame later in his career, and how acting has helped him manage his mental health.