The rules in chapter 20 are for the most part rules repeated from the last couple chapters, except this time with the addition of punishments. Yay!
The arrangement of chapters 18 through 20 is a callback to Moses getting the law on Mount Sinai. Besides puns, the writer of Leviticus enjoys the interplay of words, repetition, and physical space.
Climb the mountain in chapter 18. Face the holiness of God at the peak in chapter 19. Chapter 20 is the trip back down, learning what will happen when the rules learned on the journey are not followed. Modern readers aren't accustomed to visualizing a text like this, but The Price Is Right's cliffhanger game can be a big help.
Another way to visualize this is as a mini-Tabernacle, walking through to the Holy of Holies and back outside to day-to-day life.
Everybody Must Get Stoned
This scene from Monty Python's Life of Brian is brought to you by most of the penalties in this chapter.
God tells the Israelites to set themselves apart from the people of other nations. Just because they're wearing Uggs doesn't mean Israel has to.
Sacrifice kids to Molech? Stones. Thud. Dead.
Have a reality show about your career passing on messages from dead people? Leviticus doesn't care how high the ratings are—to the rock pile, baby.
Disrespect mom and dad? Have sex with another man's wife? Have sex with your dad's widow? Each of these carries the death penalty.
Meme-buster time. After the 2012 election resulted in gay marriage and marijuana being legalized in some states, the internet exploded with the following quote from Leviticus 20:13—"If a man lays with another man, as with a woman, he shall be stoned."
Thing is, though, the bit about stoning isn't actually in Leviticus 20:13. The penalty is stated more obliquely as "shall be put to death," with an emphasis on death.
For more details on the penalties in this chapter, check out the passages we've quoted in the discussion of the Leviticus "Sex" theme.