Avatar Korra -
Janet Varney (One of the reasons she got the role was because she was a huge fan of ATLA. Also she used to be in a relationship with Chris Hardwick who voiced Adult Sokka)
Asami Sato -
Seychelle Gabriel (She was Princess Yue in the movie. Funnily before LoK started Korra got shipped a lot with Yue so in meta way Korra still is being shipped with her.)
Bolin -
P.J. Byrne (he's Kenny Sommerfeld (MOST LIKELY TO SUCCEED!!!) in Horrible Bosses)
Mako -
David Faustino (Bud Bundy from Married... with Children)
Tenzin -
J.K. Simmons (One of Lin's fandom names is Spider-Lin and he's J. Jonah Jameson, the fandom had a lot of fun with that one)
Lin Beifong -
Mindy Sterling (Frau Farbissina from Austin Powers and Miss Briggs on iCarly)
Young Lin Beifong -
Grey DeLisle (Aside from the funny fact that she's Azula, when Lin's character was announced and everyone was asking who the father was, one of the producers made a huge rant about how that wasn't important but ended it saying "Also Lin's dad is Azula. It's complicated." That quickly became a meme so Lin's voiced by her dad.)
Jinora -
Kiernan Shipka (She started the meme
Bad Joke Kya)
Peema -
Maria Bamford
Ikki -
Darcy Rose Byrnes (She's a year older than Kiernan Shipka who plays her older sister Jinora)
Meelo - Logan Wells
Adult Toph -
Kate Higgins (She played Barbara Gordon/Batgirl in "Batgirl: Year One")
Old Toph -
Philece Sampler (Mimi Tachikawa from Digimon)
Old Katara -
Eva Marie Saint (She has an Oscar for playing Elia in "On the Waterfront")
Adult Sokka -
Chris Hardwick
Adult Aang -
D.B. Sweeney
Tarrlok -
Dee Bradley Baker (Also Appa, Momo, Nagga, Pabu and Oogi and his daughter Cora Baker voiced young Korra. Her first lines in voice acting were "I'm the Avatar and you gotta deal with it!")
Amon -
Steve Blum (Actually played a firebender in Book 3. I met him at Comic Con last year, he's just as awesome in person. I immensely regret forgetting to ask him to tell us an Amon joke.)
Amon's
boyfriend Lieutenant -
Lance Henriksen (Admiral Hackett in the Mass Effect series. Btw wtf happened to the Lieutenant?)