Question

what did john meant by "i felt my face twitch in the spasm of that idiotic grin"?

Answer 1

John was engaged with the semi visible creature in his room, suddenly his wife enters and John doesn't want his wife to catch a glimpse of the semi visible creature in his room....and in nervousness John makes certain odd movements and being nervous John's face muscle twitches giving him an idiot like appearance...
I hope you understand it well..