Text editing: Filling only one letter at a time

[J.J. McCullough]

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?

[Ann Bens]

If you uncheck Fill the letter will disapear.

Use the crop effect to reveal the letters.

Or make 3 separate letters and keyframe opacity

Many ways to skin a cat...

[J.J. McCullough]

As you can see from the photos, if I uncheck fill the entire word disappears.