According to Jeff Sneider of The Wrap, during the online podcast Meet the Movie Press, Jim Parrack whose character has yet to be disclosed, is supposedly playing Jonny Frost in Suicide Squad. Frost first and only appearance was in the Joker graphic novel by Brian Azzarello and Lee Bermejo, as a henchman and chauffeur to the Joker.
The summary of his role in the issue is telling, because it could be what will end up happening in Suicide Squad, speculation of course (ComicVine):
Jonny Frost, a low-level thug, is sent to Arkham Asylum to pick up the Joker. Joker immediately takes a liking to Frost, using him as a chauffeur. Frost drives the Joker to the lair of Killer Croc. The three go to the “Grin and Bare It” strip bar, which was formerly owned by the Joker. With the help of Harley Quinn, the Joker kills the new owner and asks the shocked audience if they are willing to help him take his city back.
This report hasn’t been confirmed by Warner Bros. or anyone attached to Suicide Squad, grain of salt will be taken until officially confirmed.
Source: Meet the Movie Press