For me, character profiles are essential. You can think of the people you want to tell your story about, give them names....but they aren't believable unless you really figure out who they are.
No one person has a single facet. So we have a life-saving, award-winning surgeon...who also works as a volunteer at the puppy shelter (aaaaaaw) Or we have the life-saving, award-winning surgeon who kidnaps and murders women in his free time (oooookaaaaaay). Just adding that one other attribute to the same character completely changes who they are, it gives them another dimension than what we first see on the surface....be that good or bad.
Once we start adding lots of these different sides to our characters, they become 'real' people. They have motivations, fears, quirks etc - they aren't just made from a cookie cutter mould, each person on the page is unique to your story.
So how do I do this?
Some writers prefer to just get to know their characters as they write, which is great - but I need something more to work with before I start. Then, when I start writing, I feel like I truly know who my characters are, and how they would believably react in given situations....as well as getting to know more about them as I write.