According to producer Bruce Timm, Batgirl appears in nearly every new episode for three reasons: Kenner wanted to do a line of toys, they wanted to take advantage of the publicity from her being in Batman & Robin (1997), and they just love Batgirl.
According to Paul Dini and Chip Kidd's book "Batman: Animated," "The New Batman Adventures" begins approximately three years after the end of Fox's "Batman: The Animated Series (1992)".
Many of the characters took on different designs for the series, most noticeably The Joker. Many fans of the previous series, "Batman: The Animated Series (1992)", had trouble accepting the new designs and as a result "The New Batman Adventures (1997)" is remembered more for the character designs than the episodes.
Roddy McDowall, the voice of the Mad Hatter, also appeared in the '60s Batman (1966) series as the Bookworm.
This is the first time that Nightwing and the Tim Drake version of Robin are used in an animated series.
"The New Batman Adventures" episodes were part of "The New Batman/Superman Adventures" programming block (along with Superman: The Animated Series (1996)) and are more commonly referred to as the "Batman: Gotham Knights" episodes.
The Riddler only has one line in the series: "Hey diddle diddle, time for a riddle!" Though, technically, he has two in the series, but the 2nd line is shared with another character in which he simply chants "Yeah!" after Harley Quinn says "We demand money!" He shares this line with Scarface. Although in a crossover with Superman, he is seen teamed with Bane and Mad Hatter. Even then, though, he doesn't say more than three lines.
This show was released in 1997, the same year of the live action film Batman & Robin (1997).