I’ve recently read/watched some books/films that have contained extended training sequences during Act I, and in each case, I found it difficult to stick with the story. I think these sequences should be avoided in most situations, unless the writer takes one of the approaches I talk about below. To be clear, I’m only referring to long(ish) sequences near the beginning, not a Rocky-style training montage in the middle or near the end.
It’s all about stakes.
If your story is a spy thriller, and your main character begins the movie without much in the way of skills, you might figure she needs to go through some kind of training before she can go out and do all kinds of cool spy stuff in the real world. But here’s the problem with spending the first 25% or so of the story this way: the stakes remain too low for too long.
If the real conflict in your story is about averting a nuclear disaster, but I have to read about your character learning to use spy gadgets in safe, controlled field exercises for the first quarter of the book, I’m bored, and I’m not going to make it to the part of your book that’s actually good. Sure, you can make the training sequence a real challenge, throw in a nice try/fail circle or two, but if your book has epic stakes, make them epic from the start.
Now, there are definitely ways you can have an interesting training sequence in Act I, or even throughout the whole book. You just need to up the stakes enough.
One good example of this is Brian Staveley’s The Emperor’s Blades. Valyn, one of the Emperor’s sons, is nearing the end of his training to become a Kettral, the most elite soldiers in the world. He’s approaching Hull’s Trial, the final test for graduation, one that many cadets fail. But even before he gets to the trial, he gets word from a dying guardsman that someone might be looking to assassinate him. Then he learns that his father has been murdered. Further attempts on his life make things even more difficult as he tries to discovering who’s after him while he prepares for the trial.
This works because the training sequence is really a complication to the real conflict, which is the threat of assassination. Of course, it’s possible to make the training the main obstacle, as long as the stakes are high enough. Maybe your main character is a criminal who volunteered to join an elite fighting force to stay out of prison. If failure to pass the training results in a life sentence, this is enough to make the reader care if MC succeeds.
I’d say this principle applies to any bridging conflict (a conflict that takes you from your opening to the introduction of the main conflict of the plot). The stakes of the bridging conflict have to be lower than the main conflict, but if they’re too low, you’ll never get the reader to the end of that bridge.