I have gotten a lot of advice about the importance of condensing your first act. Why? Because, as writers, we tend to write voluminous first acts, thinking that we need to explain everything, provide so much backstory. But you know what that does? It makes your screenplay really boring. Why? Because while you're doing all that explaining, your characters aren't living. What I mean by that is that they aren't acting. They aren't making the important choices necessary for them to change. And you probably aren't paying off the premise of the script either, which promises what we are going to see once your main character is changing -- those funny or dramatic setpieces -- and instead we remain too long in what Christopher Vogler calls "the ordinary world." So the advice I've heard is to make your first act short. If you can tell all you need to tell in 5-10 pages, wonderful! Scripts these days tend to be no longer than 110 pages, and that means you need to leave a lot of other pages open to the development of story -- not backstory.
I have come into this problem recently with the novel I am writing. I have so much set-up. And trust me, I love the set-up I've written, all 100 pages of it. Yeah, 100 pages of set-up for act one! But unless I'm planning on writing a novel that is 500 pages (which I'm not), then I need to cut a lot of that. I don't want to, of course, because I love what I've written. But something tells me that I am going to have to "kill my darlings" one of these days, whether I like it or not.
As I've explained in a past blog entry, I highly recommend working on your ending first, before you plod through so many pages of your second act. This will help you know exactly what you need to have in that first act -- all the essentials. It has also, for me at least, shed light on ways that I can cut those glorious first 100 pages I've spent so much time on polishing and thus jump into the action of act two even more quickly.
That said, I do also realize how important it is for writers to spend the time writing those first pages of their screenplay or novel during the early days of your work's conception. A lot of times, it can work as an exercise in helping you to determine the tone of the work, as well as figure out your character's voices and their dynamics. If you do have to cut a lot of those pages, though (and you probably will), you might consider seeing how you can retool some of those scenes to make them work as part of new scenes in the second act, or as flash-back throughout the second act. (Although if this is a screenplay, not too much flashback, please.)