fill in vs work 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. 

work in

verb
  • To find time or space for, amid other things. 

  • Used other than figuratively or idiomatically: see work, in. 

  • To work out using a machine in between the sets of someone who is already using that machine. 

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