While you practice or have a public performance, your facial muscles should be relaxed and calm. You will not play any better if you make faces while playing.
Your face should radiate serenity, naturalness and playing the piece with ease.
The eyebrows and the forehead should not be raised or lowered in order to follow up higher tone intervals.
At this early stage of learning how to play the instrument you should constantly have control of the above mentioned, as this will create a habit of relaxed playing.
Taken from my book