One of the hardest things about writing is shaping the characters and forming a plot.
Here is some ways to help you do this.
1. Emotion: what emotion do you want your character to promote?
For the Big Sisters the Bioshock design team wanted a broken bird type charcater trying to deal with a traumatic event that was awkward and twisted.
2. Motivation: What motivates a character to do something?
For Frank Fontaine it was for power. For many splicers it was due to a phobia for example ducky feeling paranoia.
The use of mind control is a way for an author to have a character do something that he/she normally wouldn't this was shown within bioshock with the "would you kindly" trigger.
3. Appearence: Sometimes it is best to put character development aside and focus first on what the character would look like. Inspiration can come from looking at photographs of different art styles or from another time.