I'm assuming you mean a "script-style" write on, not a "type-on" where the letters appear one by one (which can be done pretty quickly by adjusting the in/out points of each letter of the text so that they appear sequentially). The typical method to do a write on is to apply a paint operator to the image and work backwards - start from the end frame, turn on "Auto Durations" from the Object menu, erase a little bit at a time from the end of the text, go back a frame and erase some more, until you've erased the entire text. Then you can shift-click on the first and last eraser object in the workspace list and group the whole write-on animation (make sure to turn OFF Auto Durations first). You should group the final result because moving the group timebar in the timeline will let you set where your text write-on begins, plus it will let you apply effects to your write-on animation if you want. You may need go back to touch up certain frames, especially in spots where certain strokes of the text overlap other strokes.
This can be time-consuming, but I've done it myself pretty effectively on short phrases ("The End", someone's name) and I'm able to do it in 10 minutes or so. It's repetitive but not impossible.
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Greg Niles
combustion Product Specialist
Discreet