Why does McEwan decide not to have Mr. Tallis make an appearance in the story?

Mr. Tallis isn't needed for plot develpoment; therefore he's simply mentioned rather than seen.

"We never meet Mr. Tallis, distant and preoccupied man. He's an official in the Home Office, where Hitler's saber rattling can no longer be ignored and the government has begun to quietly prepare for war--a fact that slips its way into the comfortable Tallis house."

Briony's mother and father are both absent in the novel- with her mother being in bed with a headache so much of the time.

This can be considered as the primary reason that Briony fails to recognised her naivety and the power of imagination- 'her controlling demon'.

McEwan had used the absence of these characters to suggest that there was no one to teach Briony the difference between using her imagination judiciously and using it to create wild fantasy stories.