An interesting dilemma I've come across while working with self-publishing authors is that many authors have created books based on characters they know, like a daughter or a grandchild This adds a complication aspect to character development because you not only have to match a character's likeness but also their personality. So how do you do it?
Ask A LOT of questions
When developing any character it's important to understand a character inside an out. The problem is that most of the time those characters are fiction. When they're not fiction, you have to go through the same process, only this time your information on that character should be based in facts. So, ask questions. What is their favourite food? What do they do when they're bored? What are they scared of? Do they have a favourite game, colour, toy, blanket, object, etc? Getting to know small details may help you understand their personality and reflect that personality in your illustrations.
2. Ask for reference photos and not just posed, face on photos, but action shots.
Every project starts with research and the collection of photos is part of that. You can tell a lot about a person based on the type of photos they take. Are they front and centre? Are they hiding in the back? Are the tackling their friends on the football field? Plus, these images will help you with your illustrations so it's a win-win.
3. Make more than one design.
Even after going through step 1-2, I may think I have a good idea of what a character should look like but it may not be what the client has in mind. So I like to give choice, or what I call a character buffet, where I create several options that are radically different and ask the client which one they are drawn to and why or what they like about the others. We can pick and choose aspects too, like a build-your-own-character game.
Here's an example of one of those 'buffets'
I always like to learn and draw from observation but that's not always possible and could potentially be creepy so this is the method I've developed to try to make that process easier. Any other tricks to recommend?