I once had a manic depressive patient who believed she could fly an airplane. So, it temporarily does influence beliefs, especially if it makes you believe you have the answers to everything, and then you march off to implement them and convert people. A kind of Holiday Inn guest syndrome.
In the first stages of coming out of the depressed state, the patient may become rational enough to know what a mess they made of things and energetic enough to carry out a suicide plan. It is the most dangerous phase. Which is why stirring up discussion about this can be life-saving, not vicious.
I watched Mel do Hamlet, and I never saw any other actor so well evoke a depressed state of mind from Shakespeare's words. Which may have been more his life experiences at work than the timeless poetry. "O, that this too, too solid flesh would melt..."
When he said that, I knew that either Shakespeare or Mel had been there.
He can join Tom Cruise and Margot Kidder and Patty Duke among others. I suspect the acting field attracts more than its fair share for obvious reasons.