When I write a story with several characters, who are integral to the plot and will be making several appearances I find it always helpful to take notes. Sometimes these notes are merely the initial physical descriptions of the characters, because when you’re writing about a red head running through the woods and then later you reference her as blond you can lose your audience or cause momentary confusion that can be simply avoided by having note cards on hand for reference.
Other times I may write certain character traits down, just to help me keep my thoughts straight. If you are dealing with a cast of characters who span a wide ranging spectrum of personality, then you may find this helpful so that the brash macho guy, doesn’t accidentally slip up and become the whiny character for a scene. As most of us realize writing is about telling a journey of not just physical but internal, emotional journeys. These can be the more important journey, so if you are going to have your characters are going to have their personality morph into something different from what it is, that’s fine you need these to have full rounded characters. That being said it does not need to become a quick one scene change of drastic proportions, as will all things there are times where this does not apply. If you have a brave strong man face some great unbelievable monstrosity, then he may become a coward or be laid insane by the sight. In cases such as this its acceptable, but for the most part your character should grow, just as people grow physically. A character will have growth spurts but growing takes a period of time not just one quick spurt. A character who is brave will not be caught in the middle of a fight and instantly become a coward unless all his previous bravado was false. So make sure you know how your characters are going to start their journey even if the end of the road is yet unseen.
There are also times I like to jot down notes on characters’ back stories. This is an extension of the previous point about personality traits. You don’t have to know every facet of a character’s past, but if they grew up on a farm they are going to react differently than someone who grew up in the home of an investment banker. Even if they have similar dispositions and personalities, the way these two individuals were raised will change the way they think and solve problems. Know if your character hates women because of his mother, or a former love interest, or did his father instill a despising attitude toward the other sex. This is something that adds a level of depth to a character and can make them either more despised or likeable. It all depends on what you’re wanting to accomplish.
This should always be a mental note; what do I want to accomplish with this character? You don’t need to know the ending or how the journey ends, but you should know who your hero is and who is going to lead (at first) and who is going to follow (at first). There is one truth that I’ve encountered when writing fiction: sometimes the characters will write the story themselves and you just hold on and enjoy the ride. So I encourage you to make sure you know your facts about your characters and when they want to take you on a ride as opposed to vice versa, sit back and let them drive.