According to this imdb.com biography, Casey's last name is pronounced "Shy-mash-ko". After his film debut in the 1983 film Class, Siemaszko donned 3-D glasses for his minor role as a member of Biff's entourage.
The year after Back to the Future, Siemaszko played another role as a 1950s-era juvenile delinquent, in Stand By Me (1986), this time as Billy Tessio, part of Ace Merrill's gang. In the same gang was another alumnus of BTTF, Gary Riley, who is given credit as Guy #1 (more specifically, Guy #1 is the student who congratulates George for knocking out Biff, and says "Nice going!"). The movie also had Frances Lee McCain playing Mrs. Lachance.
In 1987, Siemaszko was cast as the star of Three O'Clock High. Breaking with being typecast as a troublemaker, Siemaszko played a George McFly-like role instead, as Jerry Mitchell, a nice high school student who has to stand up to a bully. Although it has become somewhat of a cult classic, Three O'Clock High was not a success. However, Siemaszko was removed from being typecast again, as in the 1992 movie Of Mice and Men, he played Curley, a character who looks to fight bigger guys, which interestingly, was the main plot of Three O'Clock High about him not wanting to fight a bigger guy but having to.
Siemaszko went back to supporting roles, in Biloxi Blues (1988), and as part of Billy the Kid's gang in the 1988 Western Young Guns. For 1989's Back to the Future Part II returned as 3-D, playing one of Biff's thugs in the alternate 1985A, and reprising his role as teenager in 1955. Siemaszko is currently a regular on the FX series Damages.