Several voice actors from Batman: The Animated Series had already made their mark in the world of Gotham long before lending their voices to the animated classic, having appeared as characters in the 1960s Batman series. Known for its campy humor, colorful villains, and larger-than-life performances, the original Batman show introduced a generation to DCs Caped Crusader in a style radically different from the darker tone of BTAS. Batman: The Animated Series honored its 1960s predecessor by casting several actors from the show, bridging the gap between two very different interpretations of the Caped Crusader.

https://screenrant.com/batman-the-animated-series-stars-original-1960s-show/