Nicole Criona of lawritersgroup.com posted this comment on my blog: “Often times, bringing in a new character will also serve as a way to avoid expository dialogue, especially in screenplays, because now the main character has a reason to explain certain things to the ‘new kid’ and thereby the audience learns as well.”
I would add to this that, often in your story, your main character is actually the “new kid” and can thus have things explained to her (and henceforth to the audience). This is especially true at the beginning of act two, when the main character has just passed over the “threshold”, as Christopher Vogel calls it, into the “new world”. In many “odyssey type” stories, the main character is now a total foreigner in a new and different world and must learn how to deal and prosper here, most often with the help of friends (and more often than not enemies). The main character oftentimes meets his/her love interest at this point – and/or what I call the mentor/motivating character – and that character can also serve to shed light on the rules of the new world and thus also inform the audience through dialogue about what is going on.
Sometimes the mentor/motivating character does this with the help of, yes, a bit of exposition. But this can also happen through action scenes, as we see the main character flounder and become flustered by this new world where the rules of her old world no longer apply. The late, famed Blake Snyder also called this the “fun and games” section of the story, where the scenes we see in the trailer of a movie come from – also where we can "have fun" paying off much of the premise of the movie. “Fun and games” means all those funny (or dramatic) moments where the main character is learning her way around in this new world, often tripping and falling on her face, much to the amusement of the audience.
If you are writing a screenplay, you can look to dedicate at least ten to fifteen pages to this part of the story. Just remember: all this good stuff you can read in books on story structure and how to write screenplays shouldn’t depended on too much by writers, or it just becomes formula. Stick to a formula too closely, and your screenplay will come off as – you guessed it – formulaic. Isn’t it boring to sit in a theater, watching some big-budget blockbuster, and you know exactly what’s going to happen because the studio execs didn’t want to take any chances on veering from the tried and tested? On the other hand, make sure your scenes emerge organically. This will happen the more you practice writing.