Yes, I think the comma is needed, and yes, in silence does stand in a sort of appositive position, though it is an adverbial.
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Anonymous"This week we will celebrate a holiday." Doesn't there need to be a comma before "we"?
"The sketch lay on the Chinese artist's table, and the artist stepped aside diffidently, in silence." My question is: should there be a comma before 'in'? What point does the comma serve and why is it there? Is it an appositive?