Well, it's two casts (BvS and
SS), although Affleck is also in
SS. And there were rumors that Eisenberg could have a cameo in
SS also.
Jason Momoa (Aquaman) was rumored to be in BvS, but he hasn't been in any cast pics. So he probably only has a cameo (post-credits?), or perhaps not even that considering Snyder's statement on the matter was something like "you will be aware of his existence".
Leto's absence is interesting. Although
IIRC, they were still shooting during Comic-Con and flew in and out quickly to make their appearance. Maybe Leto just didn't make the trip? Or maybe he's doing the method acting thing and distancing himself from the rest of the cast a little? He was sending weird gifts to his castmates...
http://www.cinemablend.com/new/Jared-Leto-Joker-Sent-Really-Creepy-Gifts-His-Suicide-Squad-Co-Stars-72256.html