If you don't know what a character will say or how they will say it exaggerate their character until you do. Keep stretching and exaggerating some characteristic until you hear how they speak. Character shows up in contrast to environment and other characters, so you only have to stretch that which creates the biggest contrast. For example, exaggerate a gangster's love of rare first editions rather than his violence: books offer a bigger contrast in his world. But for a university don- make his hobby white collar boxing.