When your characters know that they're not real, their humanity becomes more important than ever. There's already a lot of overlap between what defines a character and what defines humanity, like fears, habits and desires.
At a writing seminar a speaker once told me that female characters are more sympathetic. When I said, "Why?" he seemed to think I should calm down and told the guys not to worry because "we still outnumber the women in the room." Obviously he was too sexist to understand the point of my question: making your character human does not give them automatic humanity.
Characters in commercial fiction and Hollywood films are often systematically abused to invite an emotional response in the audience. It's annoying. It's insulting. It's bad writing. Home invasion stories are meaningless if we can't relate to the family at risk. The solution to making the audience care is not to put the character in the package of a human body. It's degrading the audience and you can't get away with it in a story that isn't about humans (like Toy Story or Finding Nemo). Writer's do it when they're lazy. Like one of my actors (Kyle Ray) once told me, "it's easier to pant than to breathe."
Just because a constituent is panting doesn't mean he's in distress. And making him the same species as the audience doesn't make him relatable. If you want to be a storyteller, it's your responsibility to fiercely guard and nurture the humanity of your constituents. In order words, make them "real characters."
One of my favorite things about film is that the actors can make this easy to do. There comes a point I hand the character off completely. "He's yours now," I tell the actor, and I count on them to be an advocate, whether they know it or not. Your actor is a real human being, and if something feels unnatural or inhuman for the character, they'll feel it. Your job is to foster a dialogue that will allow the actor to communicate that to you, even subconsciously. And as things shift in production (as they always do), an actor who is connected to their own humanity will inevitably be able to hold down the arc of the character.