Keeping in character can be challenging, depending how you are approaching their characters (voice, actions, reactions, accent, etc). The easiest way is to pull from the aspects of the character that stand out to you and used those in your writing of them.

Of course, remember above all else, that characters are only part of what makes a story enjoyable. Remember to make the plot and development of the story clear and detailed enough (show, don't tell), so that the reader can picture what your have written for the characters. They will enjoy it not only for the characters but the story itself.