Two-Face was among the more prominent long-established Batman villains who never appeared on the original Batman (1966).
Final film of Adam West. He also recorded lines for the 2017-18 season of Family Guy (1999) before his death.
Harvey Dent was drawn to resemble William Shatner as Shatner looked at the time Batman (1966) was in production. This was the same time he was working on Star Trek (1966). In parallel coincidence (or intentionally), his 'animated look' is similar to that in Star Trek: The Animated Series (1973)
Lee Meriwether's character wears the Catwoman costume in her final scene. This was because she replaced Julie Newmar as Catwoman for Batman: The Movie (1966).
Adam West and William Shatner recorded their lines separately, with West finishing his before the end of October 2016.