Actor's are very dependant. The playwright tells them what to say. The director tells them where to stand. Their agent tells them when to take auditions - heck they are at the mercy of the audition process itself. But nothing can be move affecting to the actor than the choice made for them about their clothing by the costume designer.
Imagine working on a character for weeks - in the process you are creating a fully-functioning human being. It is a very personal process drawing from yourself with the inspirations of the playwright and the guidance of the director. As you get closer to the heart of who this person is, your mental picture becomes more distinct. Just as no person thinks of themselves as a "bad person," actors can never think of their characters as "bad" even if they are playing the villain.
Then, about a week or so before you are to live as this person on the stage, you are tossed a set of cloths that do not feel right.
The pants are too big and are very scratchy - they feel like you are wearing a feed bag. The shit is misshapen and smells like it has been worn in every production of Hamlet since Lawrence Olivier was in diapers. The shoes are stiff leather that is practically worn through and would hurt your feet even if they weren't about two sizes too small. And the coat... well let us just say that the coat is the color of a swamp and is so small that you can't raise your arms over your head.
You look in the mirror and there is nothing beautiful staring back at you.
No one likes to have a bad self image, especially on a version of yourself that you have spent weeks crafting so that you are not making yourself too vulnerable. Then to look like this... it can be a very trying experience.
Of course, the costume designer is worried about her design. She's picked the pants because they are a fun texture that fits the period perfectly. The coat compliments the rest of the servant's color palate but sets you apart because you have tales. It can be very assaulting for her too having spent hours making renderings and pulling costume pieces and possible building pieces. After hours, she has the perfect collection of cloths that go together beautifully on the rack.
But the actor is the one who has to live in it. And in the end, if they don't feel comfortable, it will reflect in their performance. I know of an actress once who cried at the sight of the renderings the costume designer had done of her character. The costume was a long flowing, beautiful dress. All the renderings we masterful watercolors that could hang in a gallery as art alone. But, the designer had drawn the character's face as ugly. It was an appropriate choice for the character and for the play, but it was just too much for the actress to see herself in that light.
It is a very helpless feeling - especially for an actor who has so little control of his journey through the process. It is the truly great costume designers who understand this.