The First Order shares a lot in common with the previous Galactic Empire, including their reliance on white-clad Stormtroopers as soldiers. However, it turns out the First Order versions are very different than those that came before. Sam Witwer has done voice work for numerous Star Wars projects over the years, including giving voices to Stormtroopers in both of the recent Star Wars movies. Apparently, voicing a First Order Stormtrooper is very different than doing it for an original trilogy version of the character, because the First Order Troopers are much more emotional. According to Witwer...
It may or may not be something that you've noticed, but we can't really argue that this isn't the case. Original Trilogy Stormtroopers are totally in control of their emotions. They barely raise their voice, even when they're in the middle of a firefight. When one of them gets blasted by a rebel, the others just keep firing and barely react.
However, as Finn showed us in Star Wars: The Force Awakens, Stormtroopers in the First Order are much more emotional. Finn has a complete change of heart following his first battle where he watches another Stormtrooper fall. Later, when a Stormtrooper recognizes Finn, he screams "traitor" at him. This guy clearly took Finn's defection personally, something we wouldn't expect to see any Stormtrooper do in the original trilogy.
For a voice actor, this distinction is important, as it defines how a vocal performance needs to be handled. If you're giving voice to a classic Stormtrooper, even in newer material like Star Wars Rebels, you want to have a much more controlled voice. If you're voicing a Stormtrooper in, say, Star Wars: The Last Jedi, something that Sam Witwer would neither confirm nor deny to CBR that he was doing, you have more freedom to put emotion in your voice.
There's probably an in-universe explanation for this. The Galactic Empire was a massive organization that many people aspired to be part of. Stormtroopers were recruited from a pool of willing participants, meaning they could choose people with the temperament they wanted. The First Order is a smaller organization that gets their Stormtroopers by kidnapping children and raising them as soldiers. While this does give them the ability to try and effectively brainwash their Stormtroopers, it also means they can't really know what sort of people they have in their ranks until much later.
We'll get to know the First Order Stormtroopers, and their emotional issues, a little bit better when Star Wars: The Last Jedi debuts December 15.