(Part 1 of 2 of my extremely long answer. Please forgive haha)
Ok, let me make this clear, I LOVE MJ TO PIECES. Always have and always will. So please don't tear me up over this.
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And in this picture, you can see a pack of cigarettes on his bedside table:
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And in this interview, around the 3:18 mark, he does that smoking motion that he sometimes does, but to someone off camera. It's just my interpretation, but to me it looks like he is kind of signaling that he want to smoke with whoever it is when he gets a break. I think this because he is clearly making eye contact with someone and he his sorta nodding his head in whoever's directions.
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I most defiantly don't think he was a regular smoker or even did it for more than a few years in the 80's, mostly because there would be more evidence if he did. And these are the only 3 things I have stumbled upon after 18 years of being a fan.
To me, it wouldn't be that much of a shock if he casually smoked for a few years for the following reasons:
1. He was a musician and was always surrounded by people who smoked a lot.
2. Smoking was less frowned upon in the 80's and most people (no matter who raised you or what your opinion ends up being of it) try it at least once in their life. And MJ falls in the category of a person. He WAS a human, by the way.
3. ALLLLLLLLLL PEOPLE have moments of weakness. There is no way that he was a flawless god-sent saint every second of his life from birth till death.
4. He led a busy and stressful life and A LOT of people turn to casual smoking to relax. Like I said, it was waaayyy less frowned upon back then than it is now.
It also wouldn't surprise me if MJ went well out of his way to hide it from his fans and family. This is because he tried very hard to be a good role model for his fans, and also I get the feeling that he parents would have given birth to a cow if they found out. This is because in an interview with Janet, she is asked if she ever smoked, and she says that she did once when she was 12 and now she was probably going to be screamed at by her mother for saying that. (This interview was with Tyra last year).