Adding new elements, characters, and plot lines are vital to push a story along and keep it fresh. However, the further into the story you are the more problematic these can be. Introducing a new character in the third act doesn’t leave much room to get across their backstory, personality, plot relevance, connections to the established cast, and all else needed to make a good character. Having a new character act beside those we already care about and understand can be jarring and take away the emotional focus, confusing the audience. If a mystery tale introduced new evidence halfway through the third act that resolves the central conflict, the audience will feel cheated for investing their energies on the other clues.
The solution of using existing story elements later on is a good tool, on its own. But most writers will incidentally create those story elements and they can add them back in later to make a tighter, punchier story.
Take two friends traveling through the ruins of an ancient civilization. They turn a corner and Ben is hoisted into the air by a rope snare. Travis has to get him down somehow. If the writer just then begins to think of a way to solve this problem, then the answer is probably going to come from inside the immediate ruins. Is there a ceremonial sword on the wall which Travis can cut the rope with? Is there some symbol matching puzzle he has to figure out? But the answer could simply be the flint and steel they used to lite a fire outside the ruin entrance.
Did you anticipate the flint and steel to be used in such a way? Who cares? The answer to how do I get my friend down from this trap is use what’s around you. Just do that.
Now this might sound boring to just break out the most obvious solution, but it can be modified to have the same tension as if the characters had to search around the room. Ben could have the flint and steel in his backpack and have to wriggle it off and toss it to Travis. The rope could be out of Travis’s reach and force him to precariously scale the ancient wall carvings to get close enough. And why not throw in those spiders seen at the ruins entrance when you tried to create a spooky atmosphere (as you did the wall carvings)? Have them pour out of the gap where the rope is anchored.
To roleplay the situation, put yourself in the character’s shoes and think “What do I have and how can I use it to win?” Not to just open up the inventory like an old point and click but also what skills the characters have.
If the obstacle is a cantankerous secretary, can the swarthy airline pilot distract her with colorful stories of foreign lands long enough for the thieving heroes to slip into the offices and gather incriminating evidence? So long as that character has a backstory and a personality, they can try to distract the secretary. It doesn’t matter what you wrote for them earlier, wherever that character finds themselves, they’ll have their specialized traits to see them through.
To find these breadcrumbs throughout the story it helps to read over what you’ve already put down. Not the entire fifty or hundred thousand or more words, but passages pertaining to the current situation. The last time these two characters had an intimate moment, Joan told Clair about her mother’s troubling drinking habit but Clair changed the subject. Now having that refreshed in your mind, how’s Joan going to respond when Clair tries to talk about how anxious she is after losing her job? Instead of changing a scene because it’s too familiar to an earlier one, peel away the interpretable layers and play off them, expand upon them.
It’ll make you feel like a genius. Somehow everything you’ve put on the paper no matter how droll or conventional had a point all along. No. It’s the magic behind the curtain. What was originally just the obvious thing to have a character say or do can be made spectacular when more focused effort is applied.
You might have laid the foundation for a story about a doctor’s guilt complex over a lost patient without realizing. We see him start in a bad place in his life, find an attraction to a woman, get close to her only to discover she’s exhibiting cancer symptoms. He then questions if he was willfully ignorant of the disease in their time together and is hesitant to admit he made any mistake. So he doesn’t tell her right away, feedbacking his guilt. Maybe the lost patient was just hinted at in a conversation between the two but exploring it in later chapters provides the step one in the character arc. You might have written steps two through four, then jumped to six and seven, but that’s not a complete arc. The steps already written will read with effortless fluidity but that’s because you weren’t overthinking it, letting the natural connections breath.
Think of a story as a puzzle: make sure that you don’t already have the answer before crafting a new piece. If you have the knife then at least try to cut the rope with it. It could fail and that’s fine, but don’t punish your audience with headaches over why the characters would overcomplicate their situation rather than try the knife.