Spoiler: they did, but now they don’t.
When a Reddit user posted on the Explain Like I’m Five board asking if people actually smoked or used drugs in films, nuggets of trivia about movie drug use came in thick and fast.
One commenter pointed out that Kevin Smith was a non-smoker prior to filming Clerks, but opted to use (without inhaling) real cigarettes for his character of Silent Bob and ended up getting hooked. He then gave up smoking in 2008, only to take up weed instead after being introduced to it by Seth Rogen while working on Zack And Miri Make A Porno.