https://steamcommunity.com/sharedfiles/filedetails/?id=32736...Credit to uzura3026 on steam for making this fantastic and easy to read guide on steam of all the characters. Just going add some context with my experience using their guide.
Probably about 90% of these characters can be found in the first two chapters alone. The characters to really look out are Mentor Kristofferson in chapter 3 (his janitor and mentor variants are counted sperate, same with Mr Baylis's book store and jar clerk variants). The student in Row 4, second picture as he can only be found in the chapter 4 at Geoff's presentation at the school, and all the mall clerks/ Geoff himself in the chapter Truant blood. The bus driver at the end of Truant blood is also extremely missable but if you've done everything correct this is where your achievement should pop.