observant vs watchful

observant

adj
  • Diligently attentive in observing a law, custom, duty or principle; regardful; mindful. 

  • Alert and paying close attention; watchful. 

watchful

adj
  • Fully observant, vigilant, or aware. 

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