Yeah, the extra frames have to come from somewhere, so you have to do real Frame-Interpolation if you want that effect (re-creating frames between the ones you've got); otherwise most programs just duplicate/blend frames, which isn't quite the same thing. Keep in mind, frame interpolation works best with simple motion. It is more difficult to detect motion vectors from fast/complex movements, which can lead to warped frames. Similarly, it may be difficult to work with animation, since it is drawn, so the motion isn't real... which can also lead to warped frames. Generally, frame interpolation works best with CGI and live footage since the motion is real. It also works wonders for slowing down pans and zooms.
It's the same idea behind modern TV's that achieve 120+ Hz motion. Because you have more frames than the original, you can use the new frames to slow things down, or to make motion look smoother, depending on the final framerate used.
If you know what you're doing, this can also be achieved using Avisynth: MVTools