In a light-hearted way, I'd say that writers and illustrators don't suffer from Multiple Personality Disorder, they thrive on it. We have, through our creativity, the possibility of inhabiting many characters. Like the acting profession, we need to be able to put on the guise of protagonist, antagonist; take on a supporting role and cope with any number of extras.
The more we imagine ourselves as other beings, the better our empathy - and the better our empathy, the more creative we get. It's not usually easy for adults, though: how can we re-learn this skill?
One possibility involves The Trouser Legs of History - a Discworld concept we owe to the marvellous and much-lamented Terry Pratchett. Every small decision leads off one way or another - much like which leg goes down the trousers first. In theory, the Multiverse has a version of you that took those other decisions.
Mapping them would result in something like a delta or tree diagram - with many differing versions of 'you' at each end. It's a grand what if game you can play for yourself - or for your characters. Those key moments of a person's history, the ones they remember , are the tricky choices. Same for you, same for the people you portray (be they aliens, zombies, trolls, bunny rabbits...)
A thought about diversity
Whenever your story needs a character who is utterly different to you and your background, you need to remember The Three Rs:
Here's a for instance: when my eldest son was born prematurely, I shared the Special Care Baby Unit with many Asian mums. All the pink baby-grows were in use - but they couldn't all be girls, surely? So I asked a lady from Bangladesh. She told me pink was a fortunate colour, a happy colour bringing blessings on the wearer.
Some ideas to try out
- write an obituary for each of the people you could have been
- sketch a tree diagram of choices for your character
- improvise your character doing the round-robin 'My name is...'in front of a group - either literally, if you're brave, or to a less critical audience!