Text style-[shadow masking]  (see sample here)

1.Open a new file.In the first keyframe of layer 1 write any text eg.HELLO.Give
any font color and size of your choice.Place it in the center of the stage by selecting
the text, then Ctr+X, then Ctr+V.Go to frame 30 and press F5 to insert a frame.
Select the text,then copyit by going to Edit-Copy(Ctr+C).Lock layer 1.
2. Create a new layer.Name it layer2.Go to keyframe 1 of layer2,then paste the
text you copied exactly overlying the text of layer1 by going to Edit- PasteinPlace
(Ctr+Shift+V).Now break this text by going to Modify-Break apart(Ctr+B).Select
this text and change its color using the paint bucket tool.Then group the text by going Modify-Group(Ctr+G).Go to frame 30 and press F5 to insert a frame.Lock layer 2.
3.Create a new layer.Name it mask.Select the oval tool.Go to keyframe1 of the mask layer and draw a circle a little to the left of the text as in the picture.
Group this circle(Ctr+G).Now go to frame 30 of the mask layer and insert a key frame by going to Insert-keyframe(F6).While you are on this                keyframe move the circle so that it is now to the right of your text. Now right-click anywhere between frames 1 and 30 of the mask layer and select Create Motion tween from the options displayed.Now right-click the mask layer in the layers palette and select mask from the options displayed.
Now play your movie.
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