and he turns out to be someone who collects sports cars from the 1960s. WHAT!
Suddenly you spend an hour or more identifying sports car from his era. You know nothing about this subject. Where did he come from? Where does his hobby come from?
When this happens to you in your writing, do you slam the door on him and his research-demanding hobby, regard him as someone trying to hijack your plot? Or do you go with it, even if you must now spend an hour or more looking at cars online?
Are you suspicious about whether this is a good sidebar for your story? Do you ever wonder if you're looking for a way to go online and fool around instead of cranking out words? Is he a ridiculous diversion or a gift from your unconscious?