Optical flow

Slowing 30fps footage to a quarter speed leaves gaps where frames should be. An editor can hold each frame, which looks choppy, blend neighbouring frames, which looks ghosty, or use optical flow, which estimates how every pixel moved between two frames and generates the in-between. On clean, well-lit footage with simple motion it looks like the shot was filmed at a higher frame rate. On anything else it warps: hands smear, edges wobble, and anything crossing in front of something else tears. Shoot at 60 or 120fps where you can, and keep optical flow for the shots you cannot reshoot.