fill in vs slip in

fill in

verb
  • To inform somebody, especially to supply someone missing or missed information. 

  • To substitute for somebody or something. 

  • To complete a form or questionnaire with requested information. 

  • To beat up; to physically assault. 

  • To fill; to replace material that is absent or has been removed. 

slip in

verb
  • To include (e.g. a certain word or phrase) into a sentence discreetly 

  • To enter discreetly 

  • To play a subtle pass into someone in a goalscoring position. 

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