Facial Expression (Physiognomy)
Everything abides in man, and each movement of the soul is made felt in the entire harmonious arrangement; it results that the movement and the voice should be naturally in accord in the expression of the same thought; for example, anger is betrayed by the outward action and by the excitement of the voice at the same time. It is the same with the other emotions. The facial expression of the character traits fortifies the vocal expression, which it serves to make more striking and more persuasive. This harmony of the character and the voice is the usual procedure, that of every frank emotion. The discord between the outward action and the accent of the voice can only be the expression of a vivid emotion which one seeks to conceal: it is thus that embarrassment, lying, etc., are betrayed. Thus then, to impart to the face and the voice two different expressions would be a true misrepresentation.
Garcia makes a footnote reference attached to the heading that talks about David Garrick. The reference he makes to Grimm I have not been able locate, but I did find a page saying Grimm praised Garrick. Garcia reports:
Garrick would instantly change expression and assume the most opposite physiognomies in the shortest time which sufficed to hide himself behind a door and to reappear immediately. (See Grimm, IV, 350 – 370.)
Another footnote is attached to the body of the text:
This fundamental idea makes understood the necessity of two distinct studies which have as objectives, one the voice, the other pantomime. The facial expression shows principally in the eyes and the mouth; the exalted passions shine, so to speak, from these two points, they extend and reach the face, the head, the limbs and set the entire body into motion.
A Complete Treatise on the Art of Singing, Manuel Garcia Part 2 pages 143, 144