You need to know as much as possible about your characters, so that you can write about them effectively. You need to know what they look like, how they talk, how they walk, what they like and dislike, how they would respond in any given situation, what has happened to them in the past and what bearing this has upon their present behaviour - everything. But - and this is a big but - you don’t need to tell it all to the reader. It needs to be in your head so that you can breathe life into your character when you’re writing about them, but you also need some space for the reader’s imagination to get to work, so that they can claim some ownership of the person you have created too. In that way, they will care about them and want to find out what happens to them. Often, when I’m reading, I come upon some description that says, Annabelle ran her fingers through her dark hair and I think, but she isn’t dark, she’s a red head because I have already taken possession of her and have a picture of her in my head.

I guess what I’m saying is don’t be too literal. A physical description of somebody - he had short greying hair and smile lines round his eyes - can actually limit the reader’s own picture of the person. It is better to focus your energies on how the character behaves, how they think and most importantly, how they feel, so that your reader engages with them emotionally.

Remember to include descriptions about how other characters see your hero or heroine - this can add another dimension to their portrayal, as can the contrast between their inner point of view and what they actually say or do.