An animation of an object appears under something small or invisible to camera
From my experience text objects do not work very well with booleans (due to some letters in some fonts being non-manifold).
This is a method that can be adapted to work on anything:
You need to create a gradient mask for your material that will "stay static" despite the object movement (you do that by using object mapping and adding/subtracting the object's position from it):
Great thing about a mask is that it can be faded on the interface.
The Add node is just driven by the text
The mask is used to mix Transparent and any other shader.
If you need to place such Text somewhere you need to parent it to something first and move it through parent.
This is the result:
And the blend file:
Not quite what the OP asked..., since in the question the words appear backwards, form end to beginning of the word... (but maybe someone will find it useful...)
There is an addon to make a typewriter effect quite easily.
Get it here: http://urchn.org/post/typewriter-blender-addon
Once you enable it in the User Preferences (CtrlAltU),
Animating the text is quite easy, by changing the value on the character count.
Moving the text as it gets written you can get something like this:
To get a proper typewriter effect set your interpolation for all keyframes Constant.
There is a typing text addon here: https://github.com/meta-androcto/blenderpython/blob/master/scripts/addons_extern/animate_text.py
I haven't used it in a long time, but it still works in 2.75. The addon claims it is under the tool shelf in the 3D view, but the settings are actually under the text properties. Activate the addon and the new settings will appear at or near the bottom of the text properties.
I made a video describing basic usage. There's more, but I don't know the addon very well. I don't know how to embed on SE, so I guess a link will suffice.