I'll try to explain without going into a full out plot summary. Basically the main character joined a mercenary group, as was the character's goal. However she ends up having to run away for reasons I don't need to get into. She ends up meeting up with a new group of friends, except one. You see I had written the entire story out in fic form(or original fiction) then I decided to novelize it. Well, the characters are almost entirely out of my control now. They've changed the course of the story so much, and there's still one character to fit in. However now things have turned to the point I don't know how to introduce him, even though he'll be critical later on. The reasons they got him to join before are gone now. Is it a sign I should just scrap him and let my characters roam wild, or should I try to be a little more rigid in my plot?
you're the writer, not your characters!... stiffen that spine and take control of the runaway stage coach... do up a rough outline and see what's going on when/where, with whom... then you should be able to see which horses need to be brought under control, or dropped from the team and replaced...
Ok... well.. HOW did he get into the picture? (The answer is always a simple as we think it is) If your world is running away from you, well let it run and let it be. If john needs to ride to the rescue at some later date because she was the best he ever knew then John had to come from someplace, and that is how he enters the picture. If you feel the story bolting from the gate with the wind at it's tail, well, saddle up that horse, tilt that cowboy hat and "it and let it ride". Chances are this will be the best novel you ever wrote.
You can also just ask the characters themselves how things are supposed to pan out (how they themselves would handle the situation, for example, how this one person would be introduced). It might sound strange but trust me, a lot of the times it works. Characters are just parts of ourselves, so you're in effect asking yourself what to do. The thing is, the character is in a better position to know what HE would do. You might not even consciously know it yet, but some part of you probably knows. Just ask it like you're asking an actual person. Don't be afraid of "talking to yourself" because even though it's silly, it can get things done. Just a thought.