Stan Lee has famously said that comics is all one long second act. If you think about it, using his signature creation Spider Man as an example, his first appearance in Amazing Fantasy #15 tells the origin story with all its power and responsibility.
Nerd gets bitten by radioactive spider, nerd fails to stop criminal, nerd's uncle is killed by said criminal, nerd get religion on the whole responsibility and power thing, nerd becomes superhero.
The next time we see Spider Man, he's in high school, dealing with his issues, taking care of Aunt May and trying to find time to fight crime on the side. That's Act II of the story, for those playing the home game, and we've been there for the last 30 years.
This is why comics need (to the chagrine of hardcore fans) the occasional reboot or reimagining. You can only juggle so long before you drop a ball, or decide that adding a new ball would be great.
Some comic movies look at Act I just the way Stan tended to: make it memorable and make it felt for the entire series, but get it the frack out of the way as soon as possible. X-men is a good example of this. Despite taking the entire movie to show the team coming together, it still manages to feel like one long trailer for X-2.
Some comic movies dispense with the first act entirely. For example, Buffy the Vampire Slayer. We never see her first mission or her meeting with her first Watcher until a Season 2 flashback. In Episode 1 she is already in Act 2.
Some of the best comic movies though, take delight in that other act, the one Stan says we should ditch as soon as possible. Unbreakable is possibly my favorite superhero movie and it is all Act I, painstakingly setting up the hero's powers, his motivation, his weakness and even his arch-nemesis.
The Incredibles is the same way. While the adult heroes had their powers and were experienced heroes, as Mr. Incredible was fond of saying, "they worked alone". Over the course of the movie we see the four heroes come together as a family but also as a true superhero team.
And of course Smallville is 5 years and counting of Act I. I actually am afraid they will end the show without a single shot of Clark in the costume, which would be a huge cop-out imo.
Now there's also a few comics that have shown us a third act, such as Claremont and Byrne's classic "dark future" X-men tale that shows the death of the X-men in a future controlled by the Sentinels.
No movie I'm aware of has done an Act III supers movie yet. I think the perpetual second act is one reason why movies have fallen in love with comics a little. Movie franchises work that way to begin with and comics are as good at juggling the second act as soaps (though not quite as good as Jason movies).
I'd love to see an Act III comic movie. I think M. Night could do one about "Security" from Unbreakable.
As a writer these kinds of constructions and genre conventions fascinate me. Hope this was equally interesting for everyone else.