Norman Osborn has frequently been depicted as being insane in the comics as a result of taking that enhancement formula that boosted his physical abilities, but depending on the writer, it varies whether or not Green Goblin is a separate aspect of his personality. The first Spider-Man movie opted to go the split personality route with Norman, which is why Willem Dafoe enjoyed the role so much. Norman could present himself as normal to other people, but in an instant, he could turn into the Goblin, an unhinged super villain who had no qualms about killing anybody who stood in his way. Plus, playing Green Goblin allowed Dafoe to chew up that scenery, providing those moments of humor that Dafoe mentioned.
Article source: https://www.cinemablend.com/news/2467144/willem-dafoe-really-loved-playing-the-green-goblin-in-spider-man