If you want to challenge your plot, put the protagonist in a situation you don’t think he or she can get out of. Then, make the character find a way out. It’s like a narrative video game – for example, a first-person shooter. You run the character through until the character dies. When that happens, you go back to the last save point and try again, changing various tactics in between to try to change the outcome. At any point, you can only use the skills and tools that the character has already, and while other characters or situations might help, you don’t know that at the time.
That’s how hard situations work in a plot, too. Let’s say that as the author you have your full plot mapped out, so you know that the Best Friend swoops in unexpectedly and saves the day at the last minute. Well, you have to remember that the protagonist doesn’t know that. That means that whoever is in danger has to plan and act as if no one is coming to save them.
That sincerity on their part goes a long way to convincing the reader that the situation is serious. It also helps to have hints that the Best Friend was in a position to pull off that rescue scattered through the previous pages of the book to show that the rescue isn’t solely a deus ex machina. But the rest of making the rescue situation believable has as much to do with the villain as it does the rescuer.
You see, you can’t go easy on them. Nothing brings grumbles and mockery faster than an incompetent villain. (Do you really want the Star Wars clone comparison about aiming?) If you can make a good video game with a wimpy boss, wouldn’t it be an even better game if the boss was a challenge? If you give the villain as much depth and complexity as you do the protagonist, every bad situation gets that much harder to get out of. Your plot gets that much less predictable (wait – what? You mean the villain saw through that?).
On the other hand, don’t write yourself into a corner by making it totally impossible. If you keep running the character through, and the character cannot get out successfully no matter what he or she tries, then maybe the villain needs to be a little less impressive. Having flaws is part of realistic villains, too. Or, maybe failing at the challenge becomes a major turning point (don’t take the first person shooter comparison too literally: it doesn’t have to be a life or death situation).
Like any successful game, there has to be a challenge, and the main character has to be able to overcome that challenge with enough hard work and ingenuity – or at least, the reader has to think that’s true. Whether it’s office politics, romance, or saving the world, anything that comes too easily is less entertaining. If the character doesn’t think the stakes are high, why should we?