Text editing: Filling only one letter at a time

So sometimes I want to make animations where I fill in one letter of a word at a time. You know, like if the word is CAT, I want to make C appear, then A, then T. 

However, I find that often when I highlight one letter, and click off "fill" it "unfills" the entire word (ie, makes it disappear).  See photos. 

Now, the thing is, sometimes this doesn't happen. Sometimes I am, in fact, able to do this same thing and only "unfill" one letter at a time. Does anyone have any idea of what I am doing wrong here? What's the key step or setting I'm overlooking?