"Where" the scene takes place in an audition determines whether you stand or sit. Are you driving a car, walking down the street or sitting at a bar? If the scene is described as being in a bar, sitting and drinking and talking with a friend, the better choice would be to sit in the scene. Let the viewer know that you understand where the scene takes place. And visualize specifically what the bar looks like, so the viewer "sees" it through your eyes as you look around the room. Remember the scene starts in the middle of something, so your "sense of place" should be visualized in the few moments before you start the scene.

If you choose to stand in a scene when the description says the character is sitting, the scene would not be ruined probably, but it's best to stick as closely to the writers intent as possible so the viewer knows you understand what is going on in the scene. Otherwise, the viewer may think you haven't done your homework and are under prepared as to what is happening in the scene.

Remember you can't audition for a scene the way you will film it, so be sensible about portraying "sense of place". If you are dying in the scene on a street, you don't want to lie down on the ground. That is taking things too far. Instead, slumping in a chair will give a suggestion of "sense of place", the viewers can see you, and they will know you understand what is happening in the scene and where it is taking place.

Likewise, if you are driving a car in the scene, sitting is appropriate, but steering an imaginary car through the entire scene is distracting. Visualize you are in a car, maybe slightly turning the chair and direct your lines to the reader, not an imaginary person on your right.

In the 5 seconds before the audition starts, visualize specifically "where" you are in the scene. Visualization is your key to what your surroundings look like. If you "see" it, the viewers will "see" it.