His feature film credits included Who Framed Roger Rabbit, Tarzan, and Enchanted.
[1] He became the lead character designer for the Dutch children's television series, Alfred J. Kwak, during the 1980s.
[1] Siepermann first illustrated the title character in a 1985 poster when the theater show was running in Germany, before being used in 1987 during its performance in The Netherlands.
Siepermann's first feature film credit was for the 1988 movie, Who Framed Roger Rabbit, in which he worked as a sketch artist.
[1] He continued to work for Walt Disney Animation Studios, focusing on character design, for many of the Disney's animated features, including Mulan in 1998, Tarzan in 1999, The Emperor's New Groove in 2000, Treasure Planet in 2002, Brother Bear in 2003, and the 2007 live action film, Enchanted.