Belief Made Manifest.

"I believe, therefore it is so."
Back in the old days, clerics were given their low-level spells by servants of their gods and the higher ones from the gods themselves.
But I had a cleric who could cast spells simply because he
believed he could - he was so convinced he was a cleric (despite never actually having been ordained in any faith or specifying just which god he was supposedly a servant of) that his faith in his ability to cast spells was the reason he could cast spells, lol...
Years ago, I played a scholarly barbarian. Fights sent him into panic mode so he'd just fly in swinging. He despised combat, would have closed his eyes if he could have gotten away with it.
Feargol the Fearless came from a mighty barbarian tribe. But Feargol was more of a lover than a fighter, and a complete coward, and the thought of actually going into battle terrified him. So when the time came for him to face his first battle he was so nervous and scared about people finding out he was a coward he had
a complete mental breakdown and charged screaming toward the enemy even before the clan leaders could blow the war horn.
The rest of the tribe had to hurry to catch up to him.

Of course, everyone in the tribe mistook this for berserker rage and insane bravery, and thought he was a great warrior.
Which is why Feargol left his tribe for a career as an "adventurer". Outside of battle, Feargol is overly cautious and pretty much a complete wuss. But as soon as it's time to fight, Feargol gets so neurotic that he works himself into a complete fugue state and can't think of anything but lashing out at the source of his anguish.