Everyone says they need a voice separate from your own. That you need to know them as well as your best friend. I will be linking the two people who I found the most helpful during this process. A lot of the things I say may come up off of what they say and they might be different. After all everyone has a different way of doing things. Of course they are right they need a voice and you need to know everything about them. The very first thing I do is write down what I want each character to be like. I did this twice so far and they have evolved into something amazing.
Depending on your story idea the characters will be different. You will want something out of them. The list doesn't have to be perfect because you are writing down ideas and thoughts. For example mine are more of a person than they were when I start. Heck I added more characters to the mix. This was one of the easiest things I did. In the end you have a rough idea what they are going to be like which is one step in getting your book done. There is still tons of things you have to in order to make the character work. If you know your character really well he or she will become a baby to you. You will feel for them just like you do with a best friend and that's my goal.