I'm the DM of the group.
This is more of an acting question rather than one about mechanics.
As my group and I have progressed through our game I've introduced several recurring characters to it. After the latest game I asked my players how they felt about the NPCs’ acting. In summary they said that while every character does have its own thing going, they do show similar qualities between them, especially when showing similar emotions.
For example, there is a group of characters that are wary/angry of a particular PC. They (the players) say that while each one has its own reason to be that way, how I portray it through them is way to similar to each other.
I believe it is a problem of delivery rather than writing and while it won't affect the game mechanics I feel it does affect in the overall enjoyment of the game.
How can I show emotions differently in different characters?
We are playing using voice chat but without any video. The only visual aid I have is the virtual tabletop from the site I'm using.