John Hess of Filmmaker IQ consistently puts out informative, high quality videos on all things film. In his latest video, he goes into great detail explaining the fundamentals of a widely used in-camera effect called "forced perspective". Fair warning, there is a good deal of math involved but the process and results are fascinating.
Hess defines forced perspective as "an optical illusion that makes objects appear larger or smaller or closer or farther away by carefully controlling distance and vantage point." It's a simple enough concept but one which requires careful planning and arrangement of your scene to achieve the desired result.
Using basic geometry and trigonometry concepts, Hess explains how one can accurately determine the size relationships between two objects at specific distances and use this in practical ways to frame shots. Other considerations include finding the right focal length lens based on the field of view and focusing correctly.
Hess puts together all of these esoteric concepts with a real-world example of a "martini shot". With precise calculations captured in a spreadsheet, Hess proceeds to MacGyver together a set, get the right lens, and arrange a number of lights to get the effect just right. While you can potentially achieve something similar (or better) using a green screen and compositing in post, there is something profoundly satisfying of getting it right in camera.