Retiming
Simulator
Shoot at a high rate and there is nothing to invent: every slow frame is a real one.
Three ways to invent a frame
Slowing footage down means showing more frames than were recorded. Something has to fill the gaps, and the three common answers each fail in their own recognisable way.
Frame sampling
The cheapest option: hold each source frame for as many output frames as needed. Nothing is invented, so nothing is wrong, but movement advances in visible steps. At 40% every frame is seen two or three times.
Frame blending
Mix the two nearest source frames together. The steps smooth out into a cross-dissolve, which reads as softness on slow moves and as a double image on fast ones. Cheap, predictable, and obvious on anything with a hard edge.
Optical flow
Estimate where every pixel went between two frames and build the missing ones from that. When it works it is close to magic. When the estimate is wrong, usually at an edge crossing another edge, pixels stretch and tear.
Where flow breaks
Fast edges, things appearing from behind other things, repeating patterns, motion blur, transparency and water. All of them break the assumption that each pixel in one frame has exactly one home in the next.
Shoot it instead
None of this is needed if the frames exist. Shooting 120 fps and playing at 24 gives real slow motion at 20% with no invention at all. The costs are light, rolling shutter and file size, not artefacts.
The practical rule
Decide the slowest speed you will need before you roll, and shoot at least that multiple of your delivery rate. Retiming in post is a rescue, not a plan.