feel out vs interrogate

feel out

verb
  • To try to ascertain a person's point of view, or the nature of a situation, by cautious and subtle means. 

interrogate

verb
  • to question or quiz, especially in a thorough and/or aggressive manner 

  • to examine critically. 

  • to query; to request information from. 

How often have the words feel out and interrogate occurred in a corpus of books? (source: Google Ngram Viewer )