It really depends on the author. Some might stick very closely to the canon, while others take more liberties and make significant changes.
Often, it varies. Some authors try to maintain the core elements of the canon but add their own twists and interpretations. Others might stray further and create a completely new story within the same universe.
The main difference is the deviation from the original plot. In 'canon divergence worm fanfic', the story takes a different turn, while the original 'Worm' follows a set path. For example, a character's death in the original might be reversed in the fanfic.
The closeness varies. Certain aspects are replicated precisely, while others are adapted to fit the medium or add more drama. But the core essence of the comic is mostly retained.
It follows quite closely. There are some differences, but the main plot and character developments are similar.
It follows quite closely. The main plot and key elements are mostly intact.
It follows the manga quite closely. There are a few minor changes, but the main plot and characters are intact.
It follows quite closely. The main plot and key characters are consistent, with only a few minor tweaks for the medium of TV.
It varies. Some aspects are very close, but there are also changes made for the cinematic adaptation to appeal to a wider audience.
It varies. Some storylines are very similar, but there are also significant differences to make it more suitable for TV.
Overall, Riverdale takes inspiration from the comic but makes significant alterations to the plot and characters. Some storylines are unique to the show and not found in the comic at all.
It follows quite closely. Most of the key plot points and character developments from the comic are present in the adaptation.