This came up during today's writing session. The plot calls for one character (A) to go off and leave the other (B) behind. There's a prospect of danger for B, but they both agree it's the best way to handle things.
(I'm deliberately muddying gender so I don't give anything in the book away.)
During the writing of the scene I changed one tiny little thing: A and B always intended to leave together, but at the last minute B reveals they're not going (and explains why)
Is a little change like this really that important? Hell yes.
In the plot outline A and B knew they were going their separate ways and the reader would have got used to the idea over a chapter or two. When the characters parted it was expected, normal, boring.
In the new version, character A - and more importantly, the reader - is caught completely by surprise. B's suggestion is logical but unwanted. Conflict and tension result, and instead of the reader getting a couple of chapters to digest the idea you smash them in the gut with it in a single sentence. They don't want it, but they can see why it must be so. And that's what keeps people reading.
When plotting and writing keep an eye out for this kind of change. It's so easy to do but it's very effective.
Simon Haynes is the author of the Hal Spacejock and Hal Junior series (Amazon / Smashwords / other formats)