List of types of characters that would be good/interesting to add to the mix:
A doctor or a veterinarian (At the very least)
Prostitute/stripper or x-prostitute/stripper 
Preacher or really religious person
Country person/City person
Rich spoiled girl/boy
Someone who can not speak English very well (could be interesting)
Someone who has lost his/her arm/leg after being bitten and getting it amputated
Pregnant person
Person with a newborn child or kid
Hippie
Someone who used to be a druggie/drunk but is now going through withdrawal because s/he can't get their fix anymore
Different nationality characters (not just American or whatever, add someone who is originally from Australia or Zimbabwe or Transylvania..... This role play does take place in LA at the start. Keep that in mind. Why are they in LA? )

Be creative in the person you create!