A powerful moment on stage is when an actor is standing, not necessarily speaking, and not gesturing nor traveling in space. I was reminded of this while watching a young actor whose blocking had him stand still while the rest of the ensemble moved around him. He was connected to the stage, beautifully upright, looking ahead with intention but without the slightest bit of tension. While others may say he has beautiful posture, I say he has balance, awareness, and clarity in the moment.
When actors of olden days applied posturing to a character, it was a way
of expressing movement that fit the character's character, so to speak.
That still happens today, and you can see it in well-trained actors.
They don't call it posturing nowadays. They learn movement options,
often in an Alexander Technique class. One of those options is being fully embodied, even if standing on one's mark.
Because I am an Alexander Technique Teacher, clients often come to me seeking a way to improve their posture. When I ask them what that means to them, they usually demonstrate something very stiff. One of my students demonstrated posture by backing up into the wall and then stepping forward, using effort to maintain a sense of perfect uprightness.
Being up in space is a natural way of being. Most of the muscles and connective tissues that have this primary job are very close to the spine. Unfortunately, when people think of posture, they disregard this natural structure and apply considerable effort to superficial muscles. Superficial muscles are designed to move bones through various ranges of motion, dependent upon joint structures and the variety of forces applied in movement. When they are used consciously for uprightness, movement becomes limited and sometimes painful.
A musician who has a goal of perfect posture may do the same thing - restrict movement for the sake of an ideal position. However, a musician who understands that being upright is an internal mechanism that doesn't need that level of work will find both freedom and balance. That musician will trust being up in space to allow for the expressive and technical requirements of the instrument and the music.
Here is a photo of outstanding pianist, Vladimir Horowitz. He had a long career of playing from a place of beautiful balance. Notice how easily he moves his head while posing for this picture. If he had a concept of posture that was stiff and inflexible, he wouldn't be able to do this. I was fortunate to see him play live, and his balance and control of sound were magnificent.
Here is a photo of singer Tony Bennett, known for his smooth, expressive delivery of the American Song Book, including duets with Lady Gaga. He just retired from public performance at the age of 95. His standing balance contributes to the ease in his singing style.