While rotoscoping was initially a tool for cartoonists (in fact "Rotoscope" was once a brand name), the technique was commonly used (and still is) for special effects in live-action films.
For example, Star Wars used rotoscoping to achieve the lightsabers' glowing effect. During filming, the actors fought with sticks which were later traced onto an animation cell (frame by frame) and colored in by hand with your run-of-the-mill bright red or blue cartoon inks. A 'negative' piece of film was also created where the artist had to paint a film cell completely black where the lightsaber wasn't.
Some fortunate soul would then do the following:
1. Take a photo of the original film with the lightsaber covered by blank ink (on a seperate cell in front of the original, and manually lined up).
2. Take a photo with the inked-in lightsaber cell placed over the 'negative' cell produced earlier (for this particular effect, they over exposed the film and used a diffusion filter to create the bright glow)
3. Take a final photo combining the final images from step 1 and 2 hoping that everything had lined up properly (if not, go back and repeat from begining). Remember - the size of their 'cells' was a frame of 35mm film!
They had to perform this laborious task for all those space battles too! Nowadays everything's digital of course but the term 'rotoscoping' has still stuck.