When a big budget Hollywood film fails to satisfy, the underlying problem is often skewed story mechanics. Suicide Squad is a great example.
The foundation of what I teach is that a story creates movement, and the movement transports an audience. So a film or story that doesn't begin by going somewhere starts with a problem. The language often used to describe this kind of storytelling is a need to introduce characters. The flaw in this logic is that generally, the story doesn't begin moving forward until after all the introductions.
An example of how a film with a large cast can be done well is L.A. Confidential. It's a story about illusion, reality, and identity. Because that's introduced up front, and all the major characters have issues around identity, the opening scenes of the film both introduces the story and sets it into motion.
None of that happens in Suicide Squad. I had no idea what the story was about until deep into the film. The plot was something about a witch and her brother destroying the world and taking over. I always have the same problem with this type of scenario. If the world is destroyed and humanity wiped out, what's left to take over?
That said, at least those in the Suicide Squad have something to do now, battling faceless enemies for an obscure reason until they reach what Syd Field calls Plot Point Two, that moment in a story when all seems lost. In a typical Hollywood film, this is about 90 minutes in. At PPII in SS we finally get a suggestion for a story, that the evil doers who have survived to this point are more moral than the normal people who brought them together and command them.
That's fine, and if it had been introduced in the opening scenes of the film, the story and all those character introductions would have had a context and served a purpose. Think of a version of Rocky where you don't find out that Rocky is a nobody who wants to be a somebody until 90 minutes into the film. Think of Harry Potter (the novels, not the wretched first two movies) where you don't find out for hundreds of pages that both Harry and the Dursleys want to fit in.
The first two Harry Potter movies are wretched because like SS, all the effort goes into introducing characters and locations and it's not until deep into the films that a story emerges.
Not introducing what a story is about until PPII is a common problem for big budget Hollywood films that fail to find an audience. I actually saw a Hollywood remake of a Japanese horror film that didn't set out the point of the story until the last line of dialogue.
What this translates to for the actors involved is that since they aren't given characters to play (characters motivated by some internal purpose), the actors are left to pose in their scenes. I feel great sympathy for actors in such films, although in this case I assume they are well-paid.
All this isn't to say there isn't some fun along the way in Suicide Squad, or that a few of the characters don't make a strong impression, it just that the film never gains traction. It just slogs along.
A small point, one of the evil witches is killed by a fairly small explosive device, which could have been delivered a multitude of different ways.
And that's a big problem when the mechanics of a story transporting an audience fail: everyone in the audience has time on their hands to think about silly plot issues. Mystery Science Theater 3000 would have had a field day with Suicide Squad. The funny comments would write themselves.
The film does demonstrate another issue I come across in scripts with a multitude of characters, the writer/director ends up playing traffic cop, expending a great deal of energy just to make sure all the right characters have something to do at the right times. Bringing in studio executives to try and fix story structure problems (that they don't understand) generally create a bigger muddle of mixed tones and dialogue that never fixes the underlying problems.
I'm assuming Suicide Squad has enough of an audience that we'll see more of these characters. Hopefully the current writer/director will be promoted and someone else brought in to direct the sequel.