In chapter four, Creating Characters with Personality describes using different hair styles on female characters in order to avoid having them all look alike. The web site at this link discusses the idea in relation to manga art. Hairstyle and clothing can be very important if your style calls for less detail in the faces of characters. You never want the audience to be in doubt about which character is on screen. (Except when you do...like in The Usual Suspects. If you haven't seen that film, see it a couple of times when you get the chance.)
A character's hair can be an important identifier when the character's face is drawn in exaggerated emotion. It can show the effect of weather (wind, rain) or a level of fatigue (flatter than usual) or greater energy (brighter, shinier). A distinctive hairstyle can add to a distinctive silhouette. In general, each character must have a specific look, so make sure you have a library of styles to draw from.
Clothes need to be appropriate to a story, but also appropriate to a character. Mr. Smith remarks in The Art of Flash Animation, that an artist needs to research clothing styles, especially any clothing styles that are meant to be "modern". Styles change rapidly, so what is good for one production is not necessarily good for the next. Whether you are developing clothing for historical characters, fantasy characters, or characters meant to be current, you should look for valid sources to base your drawings on. Current styles can be the easiest to research: look over the magazines in most hair salons, like Vogue and GQ.
Finally, hairstyle and clothes should complement a character's personality. Cut and color should show something about the character, even in a still shot. The way you dress and style a character should tell the viewer something about that character even before a line of dialog is heard. Consider the character versions on the cover of Creating Characters. Mr. Bancroft went through several versions of this character before settling on the final version of Kid Kaboom. Only the first version had a cape. (Perhaps he believes Edna Mode is correct about capes...) The early smiles became smirks, which became the final version's snarl. The hair went through several styles before becoming the unruly, explosive cut of Kid Kaboom. In this version, the face and the hair fit together to show a character with an attitude.
In the last lesson, we looked at chapter seven in Creating Characters with Personality, where Mr. Corley described drawing fantasy characters. He also recommended studying the actual anatomy of real people and animals to develop a consistent anatomy for our characters. In the same logical approach, we need to make sure that the anatomy of our cast of characters fits a common "reality".
This point is illustrated in the first set of drawings in chapter eight. Mr. Bancroft's initial sketches for Carrots, Dillon, Polly, and Ruthie are shown as a cast photo. In this view, Dillon and Polly are the same size. Mr. Bancroft suggests that Polly should be shorter to look more vulnerable. No, Polly should be drawn smaller, because if she were drawn as she is in the first panel, the relative sizes of her head and Dillon's make them look like two different species. The characters must look like they come from the same reality, so they have to be close enough in scale that they look like they belong together. In the following pages, we see additional sketches of Brent, the evil sheriff, and Grit, Brent's henchman. In my opinion, their head sizes fit better with Polly than they do with Dillon. Dillon is still looking out of place. Often, when one artist controls the final look of a cast of characters, this is not a problem. Having a team of artists working on a production, however, will lead to this situation. A resolution is needed from the lead artist or from the director.