Watson will portray a character named Gay Ray, who's described as follows...
... A transgender teen who left home because of a domineering and bigoted adoptive father and has found acceptance and love in his street family. Although he calls himself "Rayna," he begrudgingly allows his friends to refer to him as Gay Ray, a nickname he's grown to appreciate. He is optimistic, funny, intelligent and a good friend.
The 12-episode third season of The Killing will introduce a new case, and will be set a year after the Rosie Larsen investigation, which dominated the first two seasons.
I watched the first season of The Killing, and was actually quite hooked; but the ending of season 1 left was disappointing, and when season 2 started, I'd lost some interest in it (partly because AMC took a bit took long to debut the second season - almost a full year between season 1 and 2, and I'd say that, by then, fans had moved on to other series).
But I might give season 3 a look when it debuts.