In Charles Dickens by His Eldest Daughter, Mamie Dickens related in the process of composition he frequently acted out the roles in a mirror: writer, actor, and critic at once:
During our life at Tavistock House [1851-60], I had a long and serious illness, with an almost equally long convalescence. During the latter, my father suggested that I should be carried every day into his study to remain with him, and, although I was fearful of disturbing him, he assured me that he desired to have me with him. On one of these mornings, I was lying on the sofa endeavouring to keep perfectly quiet, while my father wrote busily and rapidly at his desk, when he suddenly jumped from his chair and rushed to a mirror which hung near, and in which I could see the reflection of some extraordinary facial contortions which he was making. He returned rapidly to his desk, wrote furiously for a few moments, and then went again to the mirror. The facial pantomime was resumed, and then turning toward, but evidently not seeing, me, he began talking rapidly in a low voice. Ceasing this soon, however, he returned once more to his desk, where he remained silently writing until luncheon time. It was a most curious experience for me, and one of which, I did not until later years, fully appreciate the purport. Then I knew that with his natural intensity he had thrown himself completely into the character that he was creating, and that for the time being he had not only lost sight of his surroundings, but had actually become in action, as in imagination, the creature of his pen.
Reference
Dickens, Mamie. My Father as I Recall Him. New York: E.P. Dutton & Co., [n.d.] (published as Charles Dickens by His Eldest Daughter [Mary Dickens], London: Cassell, 1885, pp. 47-49:
Last modified 23 July 2012