If you pay attention you begin to notice that Vegeta actually has some Character Development in these Let's Plays. In their first Let's Play of Orcs Must Die he's very reluctant to play games with Nappa and states that he thinks that fun is for the weak. As time goes on he starts to enjoy the games more and more and even has a couple of Pet the Dog moments with Nappa. Two of the biggest examples of this are the end of the Killing Floor Let's Play, where they go back to playing Magicka and Vegeta admits that Nappa picks the best games (in hisown way) and SimCity in which Vegeta is stated to have bought the game himself.
Another example is in Castle Crashers. You can tell Vegeta was having a blast and just not admitting it.
Nappa: This is fun right? Vegeta: It's... it's a thing.
That actually seems to be a Running Gag in TSP. At the start of the video Vegeta will claim that he couldn't possibly have any fun with the game in question and almost always ends up enjoying himself.
And then they play Divekick. Vegeta is clearly enjoying himself from the moment they start.
There's a brief but touching moment in Dead Sky where Nappa tries to help Vegeta get over his fear of worms. He doesn't succeed, but it's the thought that counts.
The fact that they're working together as a team for Portal 2 without many complaints is pretty sweet.
When they accidentally disintegrate a turret by carrying it through an Emancipation Grill they both sound genuinely upset about it.
In Part 5 Vegeta admits a very rare moment he's thankful that Nappa went to college.
Nappa's singsong "We can do anything by working with each other!"
TSP Portal 2 ends on a very sweet note when GLADOS comments that the two have become "quite the team. Extremely close."
By the time he plays Broforce, Vegeta seems to have accepted it. He then goes on to thoroughly enjoy the game.
Vegeta: I have no idea what I'm looking at. And I'm okay with that!
With every horror game he plays, Krillin slowly becomes more and more confident in the face of terror. This might reflect his pluckier attitude during the Android arc, seeing as the Let's Plays appear to take place before that time.
Krillin's apology to his line gun in Dead Space part 24, Exit the Leviathan.
And then there's part 34 when he upgrades to the Level 5 suit. Each suit has more "ribs" on it.
Krillin: For her fuckin' pleasure. Aww yeah!
Krillin's triumphant laughter as he escapes Mount Massive Asylum in Outlast Whistleblower.
At the very end of Outlast Whistleblower, when the man tells the player that Murkoff will come after their loved ones, Krillin responds with, "Let me tell you something about the people that I love. They can destroy planets. I don't really think this corporation can do anything to them." There was something about that line (maybe Krillin flat out calling the Z Warriors "the people he loves" combined with the pure confidence in his voice that they could take anything thrown at them) that, even though it was only in the context of a Let's Play, this troper still found it extremely heartwarming.
His confidence is also shown in the title of the series. At the start it's "Vegeta Makes Krillin Play", but then changed to "Krillin Plays".
Seeing Krillin really get into the role of being a toddler in Among the Sleep, from playing with the toys to how he reacted to the mother character. It'll be interesting to see his later reaction when it's revealed what the child's home life was like.
For most of the beginning of Among the Sleep, Krillin has been distrustful of Teddy. So when he saves Teddy, and then thanks Teddy after he gives him some genuinely helpful advice, it's pretty sweet to see them building this partnership.