The 1966 Batman TV series on ABC was nothing less than a mid-sixties cultural phenomenon. No other TV series had ever generated more excitement than Batman. Before long Catwoman (as portrayed by Julie Newmar), became the most popular character on the show when the feline felon debuted on the 19th and 20th episodes.
The twice weekly program was such a monster hit a motion picture was filmed during the show's hiatus combining 4 super villains on one screen - The Joker, Penguin, Riddler and Catwoman. The actors who portrayed those roles on TV all returned for the movie (entitled simply 'Batman') except for Julie Newmar.
The role instead went to Lee Meriwether, as she tells the story, "I remember reading for the part. For some reason Julie Newmar, who originated the role on the series [wasn't available] - she did the first season of the series, then I did the movie in the middle the Eartha [Kitt] did the second season [the third actually, Newmar returned for season 2] then Michelle Pfiefer did the second movie, then Halle Berry played it this time... so with that you can win Jeopardy!"
Meriwether continued, "I was called in to come in [to audition] and I looked around at this room of actresses waiting to read, some very well-known, some not as well-known, or at least not to me anyway but zaftig and, you know, beautiful and I said 'Oh no, they aren't going to remember me from Adam.' So I thought I have to do something - and this is before you ever got lines ahead of time to work on."
When she did get a look at a script, "As I walked in I realized what I wanted to do - one, I changed my voice pattern, I talked up a little higher." After exchanging pleasantries, "Then, when I started as Catwoman, I dropped my voice down."
Lee Meriwether, explains what happened next that secured the part for her in this 2018 interview. Within 3 days filming would begin on Batman - The Movie.
Here's more from Lee Meriwether about her stint as Catwoman...