These characters already have voices, physical characteristics, and, best of all, something that makes them funny. When we're forced to create new characters from scratch, Dave and I gravitate to characters who have some major flaws. We like the kinds of characters who are out for themselves. Vanity appeals to us. This of course gets us in all kinds of trouble.
People read our scripts and say they hate the characters.
We then change them to lovable, kindhearted, even-tempered sorts. We don't write those characters nearly as well, so we get notes that ask us to give the characters more edge. We love stories that have a beginning, middle, and end. The stories that present themselves to us often have a beginning, but lack the other two requirements. We've learned to avoid the temptation of trying to beat such ideas into submission.