A costume can give someone permission to move differently in their own body.

Clothing changes how a body is allowed to move. Put someone in a costume and their posture, gait, and voice shift without instruction, because the outfit has quietly altered the rules they are following. The permission was always available; the fabric just made it usable.

You can borrow this deliberately. If there is a way you want to be, find the clothing, room, or setting that makes it easier rather than trying to force it from the inside. This is not pretending. It is using the fact that behaviour is contextual, which it is for everyone, whether or not they notice the props they are already relying on.