One of the most often stated reasons for enjoying drama is the dressing up, and one of the great advantages of moving away from The Station is I now have more room for costumes and props!
It is perfectly possible to do drama in the clothes you come in, and some prefer to. Obviously, this is not the case if you're making a film that day where your everyday outfit might not quite fit in with the story but certainly for rehearsals.
That said some actors find it much easier to portray characters in the costume, and only come really alive when doing so.
The costume is important it takes you out of your own persona and gets you thinking about the character your playing.
When I started working in theatre and film most productions had their own props and costume store.
Today is different with costumes rented by the day even the hour - thus not available for early rehearsals.
One lesson I try to give early is to be able to perform a character out of costume. If you're in a group session it can be good to have a "guess who I am" session with each actor playing a character without a set costume and a carefully selected script to avoid too many clues!
In theatre, the costume can be rapidly changing from scene to scene so actors should be aware that they may have to change in full view of the cast and crew.
This is again why dealing with personal body confidence and shyness early is a good idea, and that's before we even think of the makeup applications that occur.
The character can make or break a drama work - research and practice,
The costume is part of the drama process, and can really make your character whole.
One word of warning - in modern filming green screen chromakey is commonplace - and this process can take away costume, props, and actual locations. This makes it all the more important to be able to act a character out of costume.
More on this when I cover acting with chromakey.