The official website of Trek, StarTrek.com, has revealed that Doug Jones (Hellboy, The Strain), Michelle Yeoh (Crouching Tiger, Hidden Dragon) and Anthony Rapp (Rent) have been cast in Star Trek: Discovery. The casting of Michelle Yeoh was already confirmed but the character name was incorrect. Instead she will be playing Captain Georgiou of the starship Shenzhou.
Doug Jones will play the non-human character of science officer Lieutenant Saru. It is probably safe to assume that Saru will be a rather unique alien with a large amount of prosthetics to pull of the look since that is almost the only thing that Jones gets cast for. Be it Hellboy, Legion, Pan's Labyrinth, etc. the actor tends to be a supporting character covered in makeup from head to toe. Anthony Rapp will play human character of science officer Lieutenant Stamets whose expertise will be in astromycology and fungus.
Considering the show is aiming for a May release date, this is probably just the beginning of cast announcements as filming will likely need to start sometime in January to allow time for post production which will be much longer then is typical for the average TV series.