Canon alien and part-alien characters are listed in the Canon Cast page. Makeup is assumed for all alien characters.
Lead Original Alien and Part-Alien Characters
In Between Days Series
M'Roan (Jason Alexander).
Calafans name after each other, but there can only be one person with any given name (they don't have last names). Hence if there is more than one character with an exact match name, the second one is designated as II, and is, by definition, born after the death of the first person with that name. Ennewev (Alexander Skarsgard); Fepwev (Simon Baker); Linwev (Ryan Gosling); Miva (Julianne Moore); Polloria (Genie Francis); Treve (Jude Law); Yimar (Charlize Theron); Yinora (Reese Witherspoon); and Yipran (Susan Sarandon).
Quellata (Calista Flockhart).
Times of the HG Wells Series
Branch Borodin is a colony alien (Keanu Reeves); Otra D'Angelo is a human-Witannen hybrid (Keira Knightley); Kalothin is a male Vulcan (Paul Rudd); Josie (Jhasi Tantharis) O'Connor is a female Aenar (Ashley Olsen); Kevin O'Connor is a human-Gorn hybrid (John Goodman); Boris Yarin is a human-Klingon-Xindi sloth hybrid (Henry Rollins); and Yilta (Holly Hunter) is a female Calafan.
John Phlox (Liam Neeson); he's a human-Denobulan hybrid.
The Three of Us - Bithara (Penelope Cruz); Corda (Samantha Mathis); Ebrona (Courteney Cox); Izquilla (Nicki Minaj); Jeris (Kiefer Sutherland); Jobiram (Colin Farrell). Minatka (Denise Richards); Parrle (Helen Mirren); Preece Ti (Neve Campbell); Trenia (Rebecca Hall); and Vimilia (Jill Bennett).
Tre'ex (Mickey Rourke).
Extras and One-Offs Section
Klingons and Part-Klingons
Azetbur (Helen Mirren); Freela (Saoirse Ronan); Kittris (Chris Jericho); Kor (Nicolas Duvauchelle); K'Trelan (Matt Bomer); Lukara (Thandie Newton); and Rayna Montgomery (Amanda Seyfried); she is a human-Klingon hybrid.
Vulcans and Part-Vulcans
Xindi Reptilians and Part-Reptilians
Supporting Original Alien and Part-Alien Characters
Aliwev (Christian Bale); Baden (Ed Harris); Braewev (Matt DeAngelis); Chawev (Roger Daltrey); Chelben (Dawson Dunbar); Elenyakiyah (Isla Fisher); Kaiwev (Ray Romano); Yicha (Isla Fisher); Yifep (Ellen Barkin); Yipran II (Sammi Hanratty); and Yiria (Tilda Swinton).
For some alien species, there just aren't good actors or actresses, due to the need for significant makeup, prosthetics, CGI or some combination thereof. This is canon, as Xindi Insectoids, the Tholian and the Gorn in Enterprise were all CGI creations. I've had problems dreaming up people to "play" the Daranaeans, as they are canids (illustrations used for covers are all adapted from pictures of dogs, except for the photograph of Inta as a young infant, which is from a picture of a flying fox, a form of large bat. The difference in look is just too great. However, if you can come up with someone to "play" the Daranaeans, have at it. After all, M'Ress had a voice - Majel Barrett Roddenberry. Furthermore, some are just such bit parts that they should be mentioned but it's really a pointless exercise to "cast" the roles.
Fortune - Tarina (female; Freak School - Mister Kilva Tholin (male).
More, More, More! - Kerig (male); Intolerance - Legeis (female) and Lorgh (male).
Freak School - Miss T'Bek (female); Hold Your Dominion - T'Mel (female).
Reversal - She Who Almost Didn't Breed In Time; Alien Encounter - The One Who Fires a Weapon Very Fast .
Other Canon Species
The Point is Probably Moot - Dellak (female Suliban) and Glyph (male Ferengi); Freak School - Doctor Fetlaff (male Denobulan) and Tellifa (female Andorian-Xyrillian hybrid).
Fortune - Chawev II, Chelben II, Treve II and Perriwev (males) and Yidary (female); Ohio - Darywev (male) and Yimiva ( female).
The Cure is Worse Than the Disease - Elemus and Dr. Varelle; Take Back the Night - Acreon, Arnis, Craethe, Rechal, Trinning and Vidam; Debate - Boestus; The Flight of the Bluebird - Brantus and Chellis.
The Cure is Worse Than the Disease - Cama, Libba and Thessa; Take Back the Night - Cria, Dratha, Inta (there are two females with this name), Mistra, Minna and Seppa; Temptation - Jamae, Kathalia and Morza; Debate - Ethara; The Flight of the Bluebird - Anatha, Carya, Curra, Darri,Erda, Fyra, Kela, Nitha, Raelia, Samitha, Shura, Tamira and Trava.
Desperation - Chefra D'Angelo (female).
Other Non-Canon Bit Parts
The Adventures of Porthos - Dar (male Olathan) and Siont (male Azezan); Together - Griud (male Imvari); More, More, More! - Marikii (genderless Vilusun).