cave in vs play dead

cave in

verb
  • To collapse inward or downward. 

  • To relent; to grant approval against one's initial will. 

noun
  • The act of relenting. 

  • The location where something has caved in. 

  • The act of something collapsing or caving in. 

play dead

verb
  • To lie very still, like a corpse. 

  • To act as though defeated while awaiting a chance to attack. 

How often have the words cave in and play dead occurred in a corpus of books? (source: Google Ngram Viewer )