GET TO KNOW THEM LIKE THEY ARE REAL PEOPLE – In order to make them ‘real’ to your readers, you must first treat them that way yourself. Don’t just limit yourself to the information relevant to the plot of your novel, dig deeper and really get to know them, what they want, where they’ve been, their history, their philosophy and their psychology. Writing can be an unpredictable process at the best of times so you really need to know your characters inside out, to know how they’d react in any number of different situations. A great way to do this is to “interview” them – ask them deep and difficult questions to get to know who they really are.
GIVE YOUR CHARACTERS FLAWS AND INCONSISTENCIES – Nobody is perfect, after all, not even fictional characters. Or, at least, those that are are exceedingly boring, not to mention entirely unrelatable. We all have our own struggles, after all, and those of your characters will help to create internal conflict and external conflict within your story.
GIVE THEM PERSONALITY – Make them individuals. ‘The Fellowship of the Ring’, for example, would not be nearly so interesting if all nine of its members were really just copy-and-pasted versions of one single character – and how often do you come across that in real life anyway? Everyone has different traits that define who they are: some people are funny, some serious, some people are clever and some people like to act the fool, and we are all motivated by different things. For maximum plausibility, you should ensure that your cast of characters is equally as diverse.