Saw an interview with Burgess Meredith years ago.

He said that very early and during pre production of Batman, it was not overly welcomed by veteran Actors when approached. But word quickly spread that it was a hoot for the Actors. Cesar Romero was ALL in from the start and described being Joker as "A gas!"

On Youtube, there is a really cool interview with Romero. He's in make up but in a T shirt behind the scenes.