Most folks that have not shot with a real motion picture camera don't understand frame rates and slow motion. Throw in interlacing, comp frame rates, and file interpretation and it's really easy to get confused.
Let's make it simple.
The first consideration is your playback frame rate. That is usually 23.976, 24, 25(in PAL countries) and 29.97. Occasionally, and usually a waste of bandwidth the playback speed will be 59.97 or 60 fps. Digital projection is also sometimes running at 48 fps. Games can run at much higher frame rates but games really have very little to do with movies or video.
If your camera frame rate matches your playback frame rate then one second of the action you record will take one second to playback.
If you shoot 120fps footage and load that footage in After Effects or Premiere Pro and put that footage in a 24, or even a 12 fps composition then one second of action captured by the camera will still take one second to playback, you will just be skipping frames. This happens because NLE's and compositing and VFX apps like AE try and maintain real-time sync which is critical for sync sound and a bunch of other things.
If you shoot at 120 fps and change the interpretation to 30fps in AE then, no matter what the frame rate of the composition, one second of recorded action will take, do the math, 120 / 30, 4 seconds to playback. If you put your footage in a 30 fps comp or 30 fps sequence you will see every frame. If you put 120 fps footage interpreted at 30 fps in a comp that has a frame rate that is 24, or 23.976, or 60, then some frames will be blended, dropped, or duplicated. That's how it works.
Twixtor makes a good estimation of the movement of the detail areas (luminance) and predicts where things in the scene will move then creates new frames with these estimates. With the right kind of footage, this works amazingly well. With other types of footage, you have to do a ton of customizing with the controls. Twixtor can take a very long time to render and it will not perform miracles on footage that is not suitable to slow down. If your camera actually shots 120 fps then there is no need to do anything but adjust the interpretation to 30 fps or less to achieve a 4X slow-motion shot. One second of real life will take four or more seconds to playback.
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