Acting for Two: As with any animated series, you'll find some of this among the minor and one-off characters, but the most notable instance is Sean Hennigan. His main role is Decade, but he also plays the mob boss from November 11's introduction at the beginning of episode 5. This may have been a deliberate Casting Gag since the mob boss and his men are the first of November 11's victims we see, and Decade and his men are the last.
For Hei: Chinese Electric Batman, for pretty obvious reasons. In the second season, with the addition of Wild Hair and Perma-Stubble, he has been dubbed "Hobo Hei." Or "Heibo", if you're one of those informal types.
Nana Mizuki and Kate Oxley as Misaki Kirihara. Both women are known to play teenagers, young boys and girls. The latter was mentioned by several of the voice actors involved in several Darker than Black-related commentaries.
Jerry Jewell for Kastinen. Zach Bolton mentions that he was happy to have this trope for him since he portrays teenagers and young men.
Caitlin Glass as Alice Wang, who has an obsessive complex towards Misaki. She's known for voicing Winry Rockbell, Triela and Cammy.
Urban Legends: Rumors have it that the show would have a OVA that centers on what happened during Heaven's War in Brazil. It never went through.