David Ayer's Suicide Squad features a star studded cast, starring Will Smith, Margot Robbie, Cara Delevingne, Viola Davis, Jai Courtney, Joel Kinnaman, Jay Fernandez, Karen Fukuhara, Adwale Akinnuoye-Agbaje, and of course the Joker himself Jared Leto. Although most of the cast's roles were revealed during subsequent photos from the filming location and ones that David Ayer himself released, there was one actor whose role remained under wraps for most of the time during production.
True Blood star Jim Parrack was one of the actors revealed during the first photos of the cast, but his role wasn't announced together with the others. There were rumors early on that he would play DeathStroke, a well known Batman villain who has since faced off with the entire Justice League in the comics. Now those rumors are put to rest after a recent photo was released by one of his co-stars.
Now this doesn't fully confirm that he is indeed Johnny Frost, but considering it was posted by one of the actors from the film and the photo was deleted afterwards, it does seem to be quite true. This does correlate with the rumors a month ago from The Wrap reporter Jeff Sneider that according to his sources, Jim Parrack is indeed playing Johnny Frost.
In the comics, Johnny Frost is a low-level street criminal who turned into Joker's henchman after the clown prince of crime noticed his skills. He appeared in the 2006 graphic novel by Brian Azzarello and Lee Bermejo simply titled "Joker", which sounds like one of the influences for David Ayer's film. The comic featured a darker take on the famous villain, so it's not a surprise if Ayer would want to take some inspiration from the novel.