First, look at your outline and think about the dramatic plot points. What type of characters are going to make those possible? For example, Renegades required a person capable of hacking government security systems. So what was the profession of one of my characters? The developer of the government security system. I also needed my team of renegades to be able to travel freely around the star system, so I made sure that one of the people they teamed up with happened to own a ship. A lot of the time, the needs of your story will dictate the characters you have to create.
Second, make your characters believable within the context of your story. If your female heroine is a superhero or in a fantasy novel then it's fine that she has super-human strength. If she's not a superhero then you might want to make her strength more believable. If you make a character that can hack a government security system who hasn't worked in that field before, you need to explain where he acquired those skills unless readers won't take the story seriously.
Third, remember, no one is perfect. Every character, even the protagonist, must have some type of character flaw. Have you ever met a person who was seemingly perfect in real life? I have before, and I wanted to punch her in the face. No one likes those people. The reader wanting to punch your character in the face is not good (unless that's what you want, of course).
Fourth, everyone has a distinct voice. Think about the conversations you have with people. If they were transcribed for you to read, it's likely you'd be able to pick out who said what without any speech tags. Although you need to use speech tags in your writing to assist the reader, you should be able to read a transcript of the dialogue from your novel and know which character is saying what. If everything sounds the same then the characters aren't developed enough.
Lastly, know so much more about your characters than what you write in your novel. Before I wrote a word of my first draft, I had several pages of notes on each main character. I knew their birth dates, parents' names, siblings, where they lived, what their motivations were, what they studied, physical characteristics, what they enjoyed to do in their free time, strengths and weaknesses, etc. You certainly don't have to reveal all this information in your novel, but knowing each character like he/she is your best friend will help you write them more realistically.
Hope this helps with your character development! Next up: World creation!