Set costumers keep track of the costumes, making sure they get packed into the truck and unloaded safely without damaging or dirtying them. They see that costumes and accessories are distributed to the correct actors and explain proper costume care, including not eating, drinking or smoking while wearing the costumes, not wearing perfume or cologne and not being seen in the costume off the set. They establish guidelines for actors to check costumes after each use for dirt, tears and other problems, and where to place pieces when they need attention. After the show's run, they ensure that costumes are cleaned and stored, and that the costumes that were borrowed or rented are returned promptly and in good condition.
With all the costumes -- and multiple pieces for each costume -- that a theatrical production requires, it's the set costumer's responsibility to keep the production's costume continuity book updated. This book chronicles every day of dress rehearsals and performances, in order of appearance, and what each actor was wearing in each scene. In addition to making sure actors wear the exact costume when a scene picks up that they were wearing when the scene cut, the book is an accurate way to track where costume pieces were on any given day and make sure all are accounted for throughout the production. The set costumer also uses it to highlight any quick costume changes so they occur smoothly.