open one's mouth vs tell the truth

open one's mouth

verb
  • to speak 

tell the truth

verb
  • Used other than figuratively or idiomatically: see tell, truth. 

phrase
  • Used to positively assert the frank honesty of an associated statement of set of statements; equivalent to "to tell the truth". 

How often have the words open one's mouth and tell the truth occurred in a corpus of books? (source: Google Ngram Viewer )