Commercial castings will rattle through the scene you are going to do and will very much be a ‘stand there, then look there and do that’ type of direction. Listen to this direction carefully. It may be helpful to repeat it back and cement it in your mind and let the CD know you have it remembered, they can easily correct you at this stage if you have it wrong.
Set the scene in your mind, if you are in a garden - imagine it. Imagine something familiar so it is clear i.e. your own garden or living room. Take a moment before you start to place yourself there so any miming you have to do is believable. You don’t have long to show the action and feeling in a commercial, so it must be clear.
Think about: your eye-line when looking at other people/objects in the scene, camera framing (how much of a close up will depend how much you can move about and use the space), overall feeling of the scene.