In Joker, Fleck's outbursts were triggered when he became angry, embarrassed, shocked or nervous, especially in public situations. Eventually, it's an incident during a PBA episode on a train that sparks Fleck's transformation into Joker and his descent into villainy.
The Joker possesses no superhuman abilities, instead using his expertise in chemical engineering to develop poisonous or lethal concoctions and thematic weaponry, including razor-tipped playing cards, deadly joy buzzers, and acid-spraying lapel flowers.