Cadet Tilly may be one of the most dynamic characters in Trek history, the lowest person on the totem pole who's socially awkward, has a snoring issue, and is the first person to ever say "fuck" in a Trek series. She's brilliant in her realistic simplicity, one of the few characters ever who seems not just plagued by doubt as a necessity for the plot, but who just seems out of their element and trying to fit in. You know, like actual people do. It's also worth noting that a good part of the reason these characters work is ...
I love Patrick Stewart. If he wanted to go on a roller coaster with me, I would ride the shit out of that coaster until I couldn't puke anymore. But I will go on record saying Doug Jones is the single greatest casting decision in all of Star Trek. Better than Leonard Nimoy, better than Avery Brooks, better than Ricardo Montalban's chest prosthetic.
Jones is the actor who brought the Faun and the Pale Man to life in Pan's Labyrinth. He was also Abe Sapien in Hellboy and about a thousand other fantastical creatures (including Roger North in John Dies At The End, which is based on a book by Cracked's own David Wong). Like Andy Serkis, Jones excels at being the character he plays, not just playing that character. Look at how he plays Saru; he's disproportionately tall, he walks on the balls of his feet, he's thin and reedy and moves with practiced, elegant, cautious steps and gestures, like me when I'm very drunk and trying to reenact Black Swan.
Prior to Jones, only Brent Spiner brought a discernible inhuman characteristic to his role. Data was often stiff and robotic because obviously. But other prominent non-humans like Worf, Spock, Neelix, Quark, and Odo did not have any particularly difference in bearing, gestures or body language. They were aliens because someone pasted latex bums to their heads. And as awesome as Michael Dorn or Armin Shimerman are as actors, they're not the kind of actor that Andy Serkis and Doug Jones are -- the kind of actor who excels not at playing another person but playing another thing. And that's ironic, because Odo could literally be other things, like a chair or a dildo, and he still wasn't as good at it as Doug Jones.
If the essence of Star Trek is exposing humanity through use of the Other, using alien races and ideologies as a mirror, then Jones has gone leaps and bounds beyond what came before by fully immersing himself on multiple levels unlike any actor before him. Suck it, Neelix, you poor man's space Bobby Flay.