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When you think about costume designing, like most people you probably have a vision in your head of turn of the century dresses for women and the lavish costumes you see on stage and in the theater. Although this is true, there is also a more mainstream side to costume designing.
The clothes that you see characters wearing on TV and in the movies have to be chosen and put together by someone who knows fashion, and they are considered costumes. When a movie is produced the writer, director, and producer all have a vision of what the scene is supposed to look like and this includes what all the characters in the scene are wearing.
In order to put costumes or clothes together for characters you need to have an eye for fashion, and you need to be able to put the whole package together. In order to do this you may need to draw sketches to show producers what you have in mind. You also need to be able to find the clothes for the characters to wear. This could mean hitting the streets and shopping for just the right outfit.
After the clothes are chosen, a costume designer will also work with the set designers to make sure everything works together, and they will also work with the fitters and the actors to make sure everything fits properly and looks right once it is all put together.
So although costume designing can mean coming up with out of the ordinary and period clothing, it can also be a more mainstream job for a person who has a background in fashion and an eye for detail.