The Peaky Blinders actor has just completed Dunkirk, a Christopher Nolan film which stars Tom Hardy, Kenneth Branagh, Mark Rylance, and the former One Direction heartthrob.
Pictures emerged of Styles, 22, on the set of the Second World War film recently, with short hair and wearing an army uniform.
Murphy told Radio Times magazine: “Harry Styles is great.
“I had very few scenes with Harry but we got to hang out and I’ve got to say he’s a great, great kid, and really, really funny.
“Above all, Chris knows talent and would have cast Harry for a reason.
“There’s a long history of musicians moving over into acting and I come from that world myself,” said Murphy, who originally wanted to pursue a career in music.
“So I’m sure he’ll be terrific in the role. I trust Chris implicitly in that.”
Murphy is set to begin shooting series four of Peaky Blinders next year.
He said he had no idea why a drama about gangsters in late 19th and early 20th century Birmingham had struck such a chord in the US.
“I can only surmise. The quality of the writing is the main thing,” he said.
“None of us expected that this quite small show — certainly at the beginning — would have resonance over there. I mean, America mythologises their gangsters fantastically..
So (creator) Steven (Knight) was very clear that the show had that — that the working class were mythologised for a change.
“And in America they’ve been doing that for decades, so perhaps that is the connection.”