"Costume Designers are professionals who design and decide what the people performing would be wearing. They work for Television shows, films, events, etc. Costume designers are sometimes involved even at the scripting stage of the film.
They are asked for their opinions and the practicality of what extent the vision of the director can actually be brought out. Especially if the film has complex levels of dressing and costumes, like for say a science fiction futuristic film, the costume designer is asked for his opinions on how they would be taking the charge of their department and whether it's actually possible to make it happen. He or she designs the costumes beforehand and even have to take the considerations of who the actors are in the film.
The actors also might interfere in their job as per their liking, which can disturb the process. Thus, costume designing isn't merely a fashion show, but an integral part of telling a story. "