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Every once in a very long while, Lord Tywin Lannister would actually threaten to smile; he never did, but the threat alone was terrible to behold.

George R. R. Martin (2012). “George R. R. Martin's A Game of Thrones 5-Book Boxed Set (Song of Ice and Fire Series): A Game of Thrones, A Clash of Kings, A Storm of Swords, A Feast for Crows, and and A Dance with Dragons”, p.1685, Bantam
Every once in a very long while, Lord Tywin Lannister would actually threaten to smile; he never did, but the threat alone was terrible to behold.