Doc Hammer of The Venture Bros. had this reaction to himself. In the DVD Commentary to The Invisible Hand Of Fate, he says that normally he and Jackson Publick are reading the lines for their many side characters, they're just reading the lines in a funny voice without even trying to act. In that particular episode, Doc discovered his acting chops, pointing out the scene where Billy Quizboy angrily confronts the people manipulating him.
Jaleel White as Sonic the Hedgehog. White gave Sonic such a warmth and range of emotions that what would have otherwise been a prime example of WTH, Casting Agency? worked so well that he voiced Sonic in two concurrentlyrunning shows that couldn't possibly have less in common in terms of tone.
Andrea Libman has always been known for playing kiddie roles that doesn't exactly require too much acting. But her performance as Pinkie Pie and Fluttershy was great. The final two episodes of Season 1, "Party of One" and "The Best Night Ever", had the characters she voiced crack under pressure and it was outright unnerving.
This also applies for some of the other voice actresses on the show. As CR put it, their performances in the previous My Little Pony shows were probably only lackluster because they weren't given anything interesting to say.
Kazumi Evans is best known in the show as the Non-Singing Voice for Rarity and Princess Luna, and she had her first in-series speaking roles as the deliciously evil Adagio Dazzle and the cultured-to-a-fault Octavia Melody in Rainbow Rocks. But her heartwrenching outburst and emotional breakdown as Moondancer in Season 5's "Amending Fences" prove her talents go well beyond her incredible set of pipes.
While Kathleen Barr does a great Large Ham as Trixie and Chrysalis, the deeper characterization afforded to Trixie as a recurring character starting in "No Second Prances" allowed her to showcase a greater range as the character.
William Shatner proves he can do a fine performance not just in his live action appearances, but as a voice actor as well when "The Perfect Pear" aired in Season 7. Many viewers who were expecting a cop-out or parody from Shatner as he is infamous for doing in his later years were blown away by his complex performance as Grand Pear, Applejack's estranged grandfather and a damn near tragic character at that, showing a great deal of respect for the source material.
Tom Kenny has generally played a lot of comedic roles, such as Spongebob, The Mayor of Townsville, Dog, etc. It comes off as a shock when he plays Doctor Octopus, who is a legitimately creepy and threatening character, as well as managing to make you feel a bit of pity for him in a hall of mirrors when he laments "I was handsome once." in "Me Time", as well as the fact that he's about to be fired if he doesn't get Spider-Man.
When he plays Mr. Jelly Bean in Rick and Morty, he once again proves his chops. With just a few lines he manages to nail the role of a creepy pedophile rapist chillingly well.
In the SpongeBob SquarePants episode "Bummer Vacation", Tom Kenny shows how scary Spongebob can be when he's completely off his rocker, and all with just the delivery of his infamous line "I've been WAITING for you, Patrick!", where you realize that he has completely lost it and becomes Ax-Crazy, with the maniacal laughter being the ultimate proof that Tom Kenny is also able to nail the role of Spongebob becoming a scary psychopath chillingly well.
Pretty much anytime Spongebob gets emotional can count here. With a particular mention going to themovie, where you genuinley feel like he's not gonna make it.
While not as famous as his other roles, he voiced Yancy Fry, Jr., the older brother of main protagonist, Philip J. Fry. In episode "Luck of the Fryish", the scene of Yancy naming his son after his missing little brother wouldn't be nearly as effective if not for Tom Kenny's touching, completely sincere delivery.
"I love you, Phillip. I always will."
Spongebob Squarepants has Rodger Bumpass's crying breakdown in Enchanted Tiki Dreams . Poor guy sounds like he is generally miserable in his life.
Pretty much all the main cast members of the first series have fantastic emotional moments, especially considering most of them actually were kids or teens at the time of their casting. Zachary Taylor Eisen (Aang) in particular stands out for this.
Dante Basco, who's best known outside of Avatar for his role as Rufio in Hook, mostly just gets to be angry for the first season, but later on gives some absolutely brilliant performances as Zuko, perhaps his crowning moment being his reunion with Uncle Iroh in Sozin's Comet. The way is voice breaks as he tells him how very sorry he is just breaks your heart. Plus, in Season 3 we get to see more of his sweet, awkward, vulnerable side, which is always nice.
Grey Delisle has proved herself multiple times as a fantastic voice actress, but her performance as Azula in the last few episodes of the series shows off skills of a different kind. Her anguished wails of madness when Katara captures her make for one of the most heartbreaking and disturbing moments in the show despite her status as a villain.
Mae Whitman has always been excellent as Katara, but she really pulls out the emotional stops in Awakening.
Before being cast as Korra, Janet Varney was mostly known for her work on various comedic projects (such as Rifftrax). After being cast, however, viewers got to see what she could do when given dramatic substance (in particular, many critics have praised her acting in Book 4 for her realistic, and heartbreaking performance of Korra suffering from PTSD).
Steve Blum has never been regarded as a bad actor per se, but was extremely pigeonholed as the go-to, over-the-top badass kind of character. In this series, he's terrifying as Amon as well as very charismatic (while still being badass to boot), commanding the screen in every scene he was in, and both he and Dee Bradley Baker had one of the most heartbreaking moments of the entire season at the end.
Zelda Williams certainly has a lot to live up to, her dad being who he is and all. She proves well up to the task with her performance as Kuvira, giving the role plenty of both commanding gravitas and humanity.
Jeremy Shada, the voice of Finn from Adventure Time has already proven himself to be quite the capable actor, considering his age. But his performance in "Finn the Human" is even better. When Finn was under control of the Ice King's crown, his performance was disturbing.
Bill Farmer as Goofy. He's usually a clumsy, happy-go-lucky guy, but come A Goofy Movie and its sequel, Goofy shows a wide range of emotions, adding authenticity to the heartbreaking moment when Goofy finds out Max changed the map.
Jim Cummings as Tigger in The Tigger Movie. Tigger is usually a bouncy, dim-witted nut, but when he's sad, it's sad for the viewers too. His original actor, Paul Winchell, had some well played emotional moments as well (particularly in The New Adventures of Winnie the Pooh episodes such as "Stripes").
Walt Disney himself demonstrated his capacity to make mature animation with "Education for Death", a powerful and moving propaganda piece which tells how the Nazi war machine trains its expendable youth for the battlefield with, admittedly, a bit of exaggeration, but absolutely zero irony (apart, of course, from the fact that Adolf Hitler himself is the cartoon's only source of comedy). It's also the absolute most serious Disney animated short subject ever to be made.
Matt Lanter's performance as Anakin Skywalker is very popular thanks to his portrayal of Anakin as a more likable and heroic character while also showing moments that foreshadow the Chosen One's fall to the Dark Side.
James Arnold Taylor did a brilliant job with his performance as Obi-Wan Kenobi, portraying the wise and sometimes snarky Jedi Master with his own characterization that feels unique yet faithful to the other actors that portrayed Kenobi before him.
Taylor also displayed a versatile vocal range with other characters such as Plo Koon, Osi Sobeck, and Rako Hardeen (both the real Rako and when Obi-Wan disguises as Rako).
Ashley Eckstein's performance as Ahsoka Tano is very well received by many thanks to her portrayal of Ahsoka's growth from a snippy Padawan to the fan favorite she's known for today. Her performance at the end of Season Five when Ahsoka leaves the Jedi Order is almost guaranteed to make you shed a tear. Eckstein also gets bonus points for her performance as the Dark Sided Ahsoka in the Mortis arc, where she manages to make Ahsoka's brief sojourn to the Dark Sidevery frightening.
Dee Bradley Baker's performances as all of the Clone Troopers was incredible, especially since he was able to differentiate each of them with just slight variations in his voice. A good example of this is the Umbara arc, where Baker did most of the voice work for four episodes. Baker gets another shoutout for his performance in the Order 66 arc, with Fives' death from trying to reveal the truth about Order 66 considered one of the most heartbreaking moments of The Clone Wars.
Liam Neeson retains every bit of Qui-Gon Jinn that made his performance as the calm Jedi Master back in The Phantom Menace well-regarded and makes every second of Qui-Gon's cameos in the installment count.
Sam Witwer wowed many with his haunting performance as the Son in the Mortis arc. He took it even further with his awesome performance as Darth Maul and made Maul's return one of the highlights of The Clone Wars.
The LEGO Movie: Alison Brie as Uni-Kitty. Special mention goes to the scene where Uni-Kitty is crying over Cloud Cuckooland's destruction.
Deedee Magno-Hall is always heartbreaking as Pearl, and gets some of the best moments in the show because of it. We truly first got a look at this in "Rose's Scabbard", where she talks to Steven about her feelings for Rose.
Pearl: Everything I ever did, I did for her. Now she's gone, but I'm still here. Sometimes, I wonder if she can see me through your eyes. What would she think of me now?
The climax of "Sworn To the Sword" will tug at your heartstrings when she's so lost in her argument with Steven it's clear she's no longer talking about his relationship to Connie.
Pearl: NO! In a real battle, Steven won't be there to save you.
Steven: Yes, I will!
Pearl: Steven, you don't know that!
Steven: Yes, I do! If Connie is going to fight, we're going to fight together!
Connie: That's right!
Pearl: You shouldn't be anywhere near the fight! You're too important!
Steven: I'm not!
Pearl: Yes, you are!
Pearl: WHY WON'T YOU JUST LET ME DO THIS FOR YOU, ROSE!?
"Mr Greg", where she sings an utterly stunning Tear Jerker of a love ballad, "It's Over, Isn't it?" Particularly the final high note of "And she loved you, and she's gone", something improvised by her actress in the recording studio.
Jennifer Paz is consistently haunting as Lapis Lazuli, from her melancholic scenes after she's released from her mirror, to her wrath when either trapped as Malachite or admitting how she took out her hatred on Jasper in "Alone At Sea".
Matthew Moy really outdid himself when voicing Steven inside Lars's body. After that, there's also his confession of his self-loathing to Steven, when they're both being taken to Homeworld.
Zach Callison, the voice of Steven himself can have some incredible moments of pathos. His debate with Bismuth over the ethics of shattering, his obvious concern trying to heal Jasper in the moment mentioned above in her own entry, and his emotional breakdown with Connie, confessing how guilty he feels over not being able to save the above two and Eyeball Ruby, despite his best attempts to solve the conflicts peacefully. But, what really gives him kudos was in the episode "Change Your Mind" where after White Diamond rips his Gem out of him, said Gem becomes a Pink copy of Steven. Zach voices both him and regular Steven and you can really hear how much in pain regular Steven when he is trying to return to Pink Steven due to him slowly dying. But, that's nothing to the sheer power in his voice where after White asks where Steven's mother Pink Diamond is, Pink Steven replies: "She's GOOOOOOOOOONE!" with a voice so powerful that it breaks the floor underneath him.
Even more impressive in the movie and the Future miniseries, where Steven has aged up and Zach is no longer held back by using his little boy voice. It allows Steven to sing even better and get some more emotionally challenging acts shoed-in.
Steven breaking down and finally discussing his trauma with his dad during the climax of "Growing Pains" shows some serious acting chops on Zach Callison's part.
Likewise, the utter sadism in Steven's voice before he shatters Jasper in "Fragments" is utterly chilling.
Zach Callison nailed it again, during Steven's mental breakdown in "Everything's Fine". You can hear his voice filled with raw madness and despair in a brutal yet realistic way.
Nobody denies that Mel Blanc was as prolific a voice actor as they come, but there are times where he came off as just going through the motions. Then, once in a while, he'd have a moment that showed he was a damn fine actor. Notable examples:
Sylvester breaking down from bird deprivation toward the end of the Oscar-winning Birds Anonymous. Mel said that was his best work as a voice actor and producer Eddie Selzer bequeathed the Oscar he received for the short to Mel when he died, saying he deserved it for his performance (so, technically Blanc received his only Oscar for that short.)
The Ren & Stimpy Show: How can you go wrong when the two leads are voiced by John Kricfalusi and Billy West? (And eventually just West?) While Kricfalusi isn't as well-known for his voice acting, he turns in a few genuinely terrifying performances as Ren, usually when he freaks out (see his threatening Stimpy and Sven in "Sven Hoek" and his almost Shakespearean monologue when he contemplates killing Stimpy in "Stimpy's Fan Club"). West also gets in a few great performances, particularly the insane yak in "The Royal Canadian Kilted Yaksmen." West never wanted to work with Kricfalusi again after the original Spumco episodes, but boy, did it work.
Prior to his being cast as Joker in Batman: The Animated Series, Mark Hamill was best known for the heroic Luke Skywalker from Star Wars, and many people had doubts that he'd be able to do justice to such a villainous character. Today, if you ask any stranger on the street to point to a definitive version of The Joker, they'll either cite Heath Ledger's Oscar-winning performance from The Dark Knight, or Hamill. And some will choose the latter over the former.
Say what you will about Superfriends, but it was Growing the Beard during The Super Powers Team era and one of the reasons is the episode "The Fear", where Adam West gives one of his best performances in his most famous role as Batman is triggered by Scarecrow going to Crime Alley into reliving the night his parents died, and his feelings and fears about it.
Will Arnett, mostly a goofy comic actor, showing a serious amount of cred as the washed up 90s actor BoJack. Special mention to his plea to Diane to tell him he's still worth something, or his breakup with Wanda (also a SRCA moment for Lisa Kudrow).
In Season 4, BoJack's mother Beatrice has more of her backstory explored, allowing Wendie Malick to show off her skills in a dramatic role. She gives a heartbreaking performance as Beatrice, both as a little girl dealing with an uncaring father and lobotomized mother and as an old woman losing her mind to dementia.
In the Rick and Morty episode, Lawnmower Dog, Rob Paulsen voices Snuffles/Snowball. His performance of the character is most definitely noteworthy and Tear jerking during the final act, where he's manipulated by Rick into being forced to choose between Morty's life or his kingdom.
Accountant: Do you think they would've done any of this for us?
Snowball: We are not them! (Beat) We are not them.
For Donald and for Tony Anselmo's mastery of the voice. Donald has never shown a greater emotional range than he has here, when Donald's previous characterization was largely just the angry guy.
Keith Ferguson is clearly having fun voicing Flintheart Glomgold, and gives him just the right amount of jerkishness that makes him a villain we Love to Hate. He also manages to do a flawless impersonation of the Darkwing Duck villain Megavolt sounding exactly like how Dan Castellenata voiced him in the original series.
Catherine Tate, best known for her comic roles and her heart-breaking role as Donna from Doctor Who, is surprisingly chilling as Magica De Spell.
The Beagle Boys in the original series were voiced by individual voice actors like Frank Welker, Peter Cullen, Will Ryan, and Chuck McCann, among others. Here, they're all voiced by one voice actor, Eric Bauza, and he manages to give each Beagle their own identity and voice.
Kimiko Glenn and Kate Micucci really make Lena and Webby stand out in the horror-oriented "The Other Bin of Scrooge McDuck!", conveying the fear and grief of their characters during the Nightmare Sequence in a chillingly convincing way.
Bobby Moynihan as Louie is typically laid back and has some skewed priorities, but in "The Secrets Of Castle McDuck", we see more of his vulnerable side when he's just as shocked and hurt as Huey that Dewey kept a secret about their mother from them. Damn! That's fine acting there.
While Jim Cummings is usually great, he's especially so here as Jim Starling, the original Darkwing Duck. What clinches it, though, is his terrifying portrayal of Jim after he decides to become Negaduck, really selling the FaceHeel Turn.
James Monroe Iglehart's previous work for Disney included the Genie in the original Broadway cast for Aladdin, Lance Strongbow in Tangled: the Series, and Bronzino in Elena of Avalor, all heavily comedic characters. So not only was casting him as Taurus Bulba a major case of Playing Against Type for him, but he gave a crazed, sinister, truly threatening performance to completely justify it.
Sarah Silverman may not have been the most family-friendly performer with a Disney film (Wreck-It Ralph) on her resume, but the way her character Vanellope cries out when Ralph destroys the car they had builtwill break your heart.
Jesse McCartney's performance as Robin/Nightwing in Young Justice. DC fans were skeptical at first of a former teen pop star voicing the iconic protege to the Dark Knight. However, they later found out he does really well balancing Dick's snark and silliness right alongside his drive, determination, and the fact that he was trained by Batman.
In "Summerween" guest voice actor Jeff Bennett gives an otherwise ridiculous villain-A cryptid made of rejected candy called "The Summerween Trickster" a genuinely menacing and suspenseful performance, making it one of the seriess creepiest characters.
Jason Ritter gives a gut wrenching Anguished Declaration of Love in "Into The Bunker" when he finds an injured Wendy. the fact that its the Shapeshifter in disguise and the real Wendy is fine doesnt take away how heartbreaking it is.
Dipper: And I should trust you why?! After you stole radioactive waste?! After you lied to us all summer?! I DON'T EVEN KNOW WHO YOU ARE!
Mabel: Grunkle Stan....I trust you.
The Grand Finales epilogue has the saddest performance that J. K. Simmons has possibly done. When he breaks down as he hugs the mindwiped Stan, you can hear the grief and guilt in Fords voice.
Ford: You're our hero, Stanley....
Sammy Davis Jr. in Heidi's Song. While no one can deny Davis Jr.'s skill as a performer, he was usually known as a charismatic showman. Here, he plays a villain, and you can tell that he's really into it. The way Head Ratte's voice rasps in certain lines (most notably, "Unless your style / is so vile / you can cause a sensation) is just incredibly well done and very sinister. Despite that, Davis Jr. still brings a lot of charisma to the character, making the Head Ratte a very enjoyable antagonist.
Big City Greens has Chris Houghton realistically burst into tears in the episode Phoenix Rises . He makes you feel that Cricket is overcome with guilt and sounds actually worried that his dog ran away for good. It is worth noting that Chris was actually in tears when recording that scene.