I have gone through the Twixtor tutorials (though I'm pretty new to After Effects), and I can't seem to figure out the trick of using a solid to extend the length of a clip. I insert both the clip and the new solid as separate layers, with the solid on top. I click the eye and solo on the solid and de-select it on the clip layer. Then I add the Twixtor effect to the solid, with the color source as the clip. I choose my new speed (say 25%), and then...nothing. I just get a big red square in my viewer. It's a big blank of red color. No picture. No slow-motion. I wonder what I'm doing wrong?
Thanks. You were right. The footage was 720x480, even though it was "squeezed" down to 4:3 in a 640x480 composition. So I made the solid 640x480 too. But in fact the solid needed to be 720x480, just like the footage.