I checked out this interview with comics artist Kevin Nowlan talking about his experience in designing the characters in BTAS, before they were refined and polished once the show aired on TV.
https://web.archive.org/web/20201111200939/https://13thdimension.com/kevin-nowlan-lifts-the-cowl-on-his-batman-animated-designs/
As Nowlan says, he had to follow the objective of drawing as few lines on the characters as possible, and looked at the Adam West TV show as inspiration for the likes of Penguin and Riddler.
It's important to note that Nowlan and Timm both agreed that drawing such high detail like in the comics would've been too difficult to adapt to animation. Some people may argue the X-Men animated series succeeded in this area, but even that show had lots of colour rendering inconsistencies and still doesn't quite look as impressive and artistic as BTAS.
Nowlan's work may have been revamped, but you can clearly see Robin and Killer Croc (except for the alterations made to his teeth) closely resembles the show's final product. Special mention should go to Nowlan's Catwoman design, you can see where TNBA's inspiration for the character revamp came from. Timm definitely made a fair amount of refinements for Nowlan's Joker design too.
Guess I didn't realize Kevin Nowlan had design input on BTAS. Obvious tho, in retrospect. His influence remains even tho the exact designs were heavily modified before production.
Very interesting, good find.
They really had an all-star collaborative team onboard. You can still see a lot of his direct influence in some of the final designs. Ironically his more minimalist style closely aligned with the TNBA redesigns, albeit with exaggerated proportions now.
Check out this video taking an in-depth look at more character and background designs for BTAS, including Bruce Timm's animation guide on how to depict Batman's body movements.
https://www.youtube.com/watch?v=x7i0gtLnF40&t