Star Trek changed the look of the Trills due to Terry Farrell
Star Trek: The Next Generation debuted an alien race known as the Trills. A symbiotic alien race that has a humanoid host, and a slug-like symbiote that is passed down from one member of the race to another upon death. Once the symbiote bonds with their host, they’re usually paired for life. The host will even take on the symbiote’s name in conjunction with their own. When debuted, the aliens had a slight forehead ridge, not too major, that is until Deep Space Nine cast Terry Farrell.
When Farrell was cast in the role as Jadzia Dax, the next host in the line of the Dax symbiote, Paramount and the rest of the Star Trek: Deep Space Nine producers realized the look of the Trill didn’t work for Farrell. While talented, Farrell was given the role in part due to her looks. Looks that producers and executives felt would be diminished by the prosthetic she’d have to have on her head.
Famke Janssen may have inspired the Trill look of Terry Farrell.
So the decision was made to re-design the Trills. Instead of going too far out of the box, executives decided to just repurpose another alien race’s look. Enter Famke Janssen. Janssen had a little role on Next Generation as Kamala, a Kriosian. The Kriosian had a specific look that included almost leopard-like spots alongside the edge of their faces.
As the story goes, Janssen was a leading candidate to play Jadzia Dax on Deep Space Nine, and despite not taking the part. Whether directly or indirectly, the studio designed Jadzia Dax after Janssen.
The producers were happy with the move, even if there was little to no difference between the two alien races upon Dax’s first appearance on DS9. Ironically enough, Enterprise would bring back the Kriosian’s and not really do much to change the look of them, other than to reduce the spots.