When the Yogscast started their Minecraft series (speaking only about the main episodes, not the side episodes like the mod spotlights), it was created as a Let's Play of Minecraft (complete with showing how to survive the first night). It eventually evolved into one of the most well-produced machinima series ever made.
Also, Lewis had never played the game before, so Simon did most of the work. After Lewis became more familiar with the game, the role was flipped, and Lewis usually took charge.
The first season of Red vs. Blue had the cast use voice filters to try and simulate the effects of helmets on their speech. This was removed in the second season since it made it difficult for viewers to understand them.
Church originally was a ghost rather than an AI, so early on, especially before they decided to actually go beyond about 10 or so episodes (which was the original plan), Church was capable of doing things that he could not do in later seasons.
Grif commonly tells Simmons that they should get back to work and focus on their tasks in the early episodes. The Grif of the later seasons is anything but concerned about tasks, reveling his status as team slacker. He also is later claimed to be the sole draftee of the Red and Blue armies, but in the first episode claimed to have "signed on to fight some aliens".
Sarge had a voice akin to R. Lee Ermey initially, but his voice actor Matt Hullum found it too hard to keep doing, so he traded it in for a stereotypical Texan accent.
Caboose was just kinda dim when he was first introduced, not the completely divorced from reality character he is today. Midway through Season 1, his intelligence began dropping in chunks between episodes, and has never come back. Word of God is that being possessed by O'Malley then having him being forcibly ejected left a detrimental impact on Caboose's head.
Donut was just a red counterpart to Caboose, also being vaguely dim. He also objected to his pink armor, calling it "lightish red". Once Season 2 came, the writers decided they couldn't get much material out of that angle anymore, and instead made him a flamboyantly over the top Camp Gay character.
Master Chief is actually mentioned in the first episode—him "blowing up the whole Covenant armada" is given as the reason why Red and Blue Armies are now fighting each other, because they have nobody else to fight. The Halo games are never referenced directly again, although the latest couple of seasons have started to pull the series back around to fitting into Halo continuity with mentions of the UNSC, Insurrectionists, and explanations of AI that fit with the Halo explanations.
They go so far as to have the war between the Reds and the Blues be black ops meant to train their brand of supersoldiers (the Freelancers) against the Covenant, though the operation was a dismal failure.