To start off, here are some of the older main articles Harry's been mentioned in (not counting pic-spam articles):

Pre-production interview w/ casting director John Papsidera about Harry's casting - excerpt:

John Papsidera said: “Harry was absolutely right for the role. As an actor he is unknown, but his readings made him an obvious choice. We thought he was fresh and interesting and he won the role.

“It wasn’t because he’s a well known pop star – if anything that was more of a detriment, because it could bring the wrong message and we don’t want people pulled out of the film because of who they are.

“Our aim was to have people who are fresh and don’t bring a lot of prior work with them, so the audience will be really immersed in the film and not struggling with recognisable famous personalities.

“So Harry really had to overcome hurdles to get past that. He fought hard for it and his work was impressive and that’s what attracted us to him.

http://www.mirror.co.uk/3am/celebrity-news/harry-styles-tipped-hollywood-success-8072670

In article about Fionn

He (Fionn) was put through the ringer before production even started. “I was quite scrawny when I started out, so they saw that and realized that they might injure me in the whole process of shooting,” Whitehead laughed. So the upstart was dispatched to Dunkirk (where the majority of filming would be completed on location) two weeks early to work with the stunt team. Along with costars Harry Styles and Aneurin Barnard, Whitehead was put through a boot camp of sorts. “I did a lot of circuit training, did a little bit of boxing, did some weapons training. Then I went to the beaches and I was swimming in full war gear, which once it got waterlogged was about 60, 80 pounds. Running up the beach with stretchers with weighted dummies on them. It was quite a lot.”

Speaking of Mr. Styles… The One Direction singer also makes his film debut in Dunkirk, and Whitehead had nothing but props for the pop star-turned-actor. “He’s a lovely guy. Really hard working. There was no preferential treatment, and he didn’t ask for any. He was just a great asset to the team, one of the crew, no differentiation.”

https://www.yahoo.com/movies/meet-fionn-whitehead-the-lead-of-christopher-nolans-highly-anticipated-battle-epic-dunkirk-023210616.html

Hardy about Styles:

Dunkirk, which stars Hardy as a spitfire pilot, also stars young One Direction singer Harry Styles in his first film role. Unfortunately, the pair didn’t really work together on the set. “I met him once, really briefly. It was lovely, big hug,” Hardy says. “He’s very polite and just a sweet guy.”

Nolan, however, was “very effusive” about Styles, Hardy continues. “And that’s not easy, to come in from a very specific department in the industry, and cross over. So poor guy is gonna be judged. I wouldn’t want to be in those shoes, but from what I hear is that, actually, it’s gonna be a very comfortable transition for him, because he’s done such great work—which is really a testament, not only to his skill as an artist, [but] also Christopher’s part in that as well. . . . One Direction aside, do you know what I mean? Which I can’t say I know an awful lot about.”

http://www.vanityfair.com/hollywood/2017/01/tom-hardy-taboo

In interviews for other projects, some of the stars were asked about Harry (rude IMO but the celeb media is relentless). Cillian said Harry had a lot of charisma and that he (Cillian) understood the path from performing live music to acting because that was also his path. Jack Lowden said Harry went through the same audition process he went through. Mark Rylance said Harry fit right in, didn't have any airs, was well-liked by the stunt crew, and that he thought Harry was brave when doing some uncomfortable stunts.