Director Scott Cooper talks about Dakota at the AFI Fest Q&A of "Black Mass" [November 8th, 2015]

[About the Cast] The Ensemble In “Black Mass”

Scott Cooper: When you cast Johnny as the lead, you do get a lot of leeway to really cast great actors. [...] I had never seen Dakota Johnson’s work, Francine [Maisler, casting director] recommended her. I typically don’t like to audition actors because, as a former actor, I know it’s the least creative endeavor. You typically see the casting director with terrible lighting, they don’t even listen to you. it’s really a terrible way to cast your film, so I typically spend time with actors and I spent time with Dakota. And I said, "Wow, she’s such an old soul that I know she can elicit a very tender side of Jimmy Bulger." 

You can read part of the Q&A here.