Try copying and pasting this on your layer:
Adobe After Effects 8.0 Keyframe Data
Units Per Second 30
Source Width 1
Source Height 1
Source Pixel Aspect Ratio 1
Comp Pixel Aspect Ratio 1
Text Animators Animator #1 Selectors Range Selector #1 Start #4
Frame
-1
Text Animators Animator #1 Selectors Range Selector #1 Offset #6
Frame
0
Expression Data
charPerSec = 10;
idx = (time*charPerSec);
End of Expression Data
Text Animators Animator #1 Selectors Range Selector #1 Advanced Units
Frame
2
Text Animators Animator #1 Properties Opacity
Frame percent
0
End of Keyframe Data
It will add an opacity textanimator with an expression on its range that keeps up with the sourceText getting longer. You’ll need to make sure the charPerSec is the same in both expressions.