Doing a voice, let alone a new voice, can be difficult. While I might come up with one I like it's not always easy to reproduce it the next day. My throat might be tighter, drier or more congested than when I first did it. And I've got to find the right voice for the character. It's also tricky switching between voices. Eventually, I get use to doing the voice enough so that I can keep it fairly consistent.
I've also got to do a voice that doesn't sound too much like any other. When I first tried to come up with a voice for the new character I was going to use a Peter Lorre type voice but that came across sounding too much like Charlie. What I came up with was something between Walt and Charlie that sounds distinct enough.