Apart from an original concept design, such as for a fantasy world or a specific time period piece, much of costume design involves styling; pulling for characters set in modern day clothes (pulling in this post means buying, renting, getting clothes together etc. for the shoot). This is similar to Fashion styling, so how do they differ? Instead of puling the latest trend, high end designer or avant garde clothing, as one would in fashion styling for a red carpet event or photoshoot, costume design focuses on the character, and bringing that character to life. Some of the questions I would ask myself when doing costume design could be: Does this character have any particular ticks or habits? Are they stylish, frumpy, or nerdy? What would their economic situation be? All these details help me to pick what the character as a real person would actually wear, rather than just make them stylish, because their character might involve to not look stylish. One area of Fashion styling that can be similar to costume design, is in some of the magazine spreads you might see, where a story is being told or where there is a theme. What we wear tells our story, it tells the world something about who we are. As a costume designer and a stylist, my job is to subtly tell a story with their clothes, and to help the actor to really feel the part, and to believe that they are the character.
I like how you say that costume design differs from fashion styling because a costume designer takes the character into account. Considering how a character would dress makes sense because it would add legitimacy to the piece. This would be especially prevalent if it is a period piece and that is part of the aesthetic of the film.