You get a free character slot for each faction. They have each their own story up to a certain point, so you might want t play them all through at least up to the end of their respective unique missions.
Compare their mission journals to find out which ones those are.
Finding the accolade is not hard, it's pretty much common knowledge, but I won't spoil it at this point.
Besides Dorn and Crosby there are Voice Overs by Leonard Nimoy, Zachary Quinto and Chase Masterson in the game.