In A History of English Literature, Robert Huntington Fletcher writes about characters. Here's what he says:
CHARACTERS. There is, generally speaking, no greater test of an author's skill than his knowledge and presentation of characters. We should consider whether he makes them (1) merely caricatures, or (2) type characters, standing for certain general traits of human nature but not convincingly real or especially significant persons, or (3) genuine individuals with all the inconsistencies and half-revealed tendencies that in actual life belong to real personality. Of course in the case of important characters, the greater the genuine individuality the greater the success. But with secondary characters the principles of emphasis and proportion generally forbid very distinct individualization; and sometimes, especially in comedy (drama), truth of character is properly sacrificed to other objects, such as the main effect. It may also be asked whether the characters are simple, as some people are in actual life, or complex, like most interesting persons; whether they develop, as all real people must under the action of significant experience, or whether the author merely presents them in brief situations or lacks the power to make them anything but stationary. If there are several of them it is a further question whether the author properly contrasts them in such a way as to secure interest. And a main requisite is that he shall properly motivate their actions, that is make their actions result naturally from their characters, either their controlling traits or their temporary impulses.