I find that - while okay for minor characters - if you're having to do a notebook about who your main characters are and what their personalities are, then it's always going to be a bit forced. The main characters are who the audience are going to be spending most of their time with, and they need to be so much more than just a put-together backstory and personality. They need to be truly organic, and for them to be at their best you pretty much need to believe they're real. Unless you're someone who needs a notebook to remember your friends' backstories and personalities, you shouldn't need one for your main characters.
Also, one shouldn't be afraid to put bits of themselves into a character. Not what you want to be, mind you - that is just going to look like a fanfic-style self-insert - but who you are. Your insecurities, your fears, your view of the world (but make sure to treat that last one objectively, as otherwise it will just come off as preachy). You don't need to put all of yourself in there, but I think an element of it being personal to you is important. After all, you're (usually) the only person in real life whose innermost thoughts you know, so when writing someone else's innermost thoughts yours are the only reference point. This doesn't always have to be planned, either - I find most of my protagonists seem to have naturally picked up my fear of growing up, and a general sort of confusion about sex.
Something else I think people never take seriously enough when writing is dialogue, though this may be more down to my own writing style. I generally find that - while backstory can be explained through narrative exposition - the only way to truly bring out a character's personality is through conversation: what they say, how they say it, other things they do while saying it.
I am also a great stickler for natural dialogue, but I think I may be in the minority there, as the vast majority of famous books, films and TV shows have surprisingly unnatural dialogue (that's not to say the dialogue's bad, but they often don't actually talk anything like real people). In real life, people don't usually know what they're going to say next, so conversations are rarely simply back-and-forth. People often stutter or pause - I've heard it suggested that putting in every "um" or "er" breaks the flow of the dialogue or whatever, but in real life dialogue rarely flows, and it's still easy enough to follow. I find that if your characters talk like real people, it becomes easier to accept them as real, and not just as characters on the page.