It's early for me, haven't had much tea yet, but I'll whack this pinata.
Probably like most of the world outside of Australia, I first got to "know" Mel Gibson in a movie called
Gallipoli, which was a sad but achingly beautiful film made in 1981. He played a tragic, achingly beautiful man with stunning conviction and depth. He was also nude for a beach/swim scene which was my main motivation in seeing the film in the first place, and remains my chief memory of it today.
If memory serves, his penis is briefly visible in a running-for-the-water shot (though his ass was much more prominently featured): it was just a very ordinary, flaccid cut cock. He built his career, in fact, on his extraordinary good looks and exceptionally fine ass; it certainly wasn't on any outstanding acting skills. In that regard, he was always much more a "movie star" than any real "actor", and perhaps the best of that breed we had throughout the '80s.
The crazy came much, much later. I'm sure alcohol has played a major role not just in his having lost his looks but in his public raging as regards women, Jews and gays.