Short answers, pulled from the story.
Sméagol killed his friend Déagol to take the One Ring from the River Anduin during the Second Age. Over centuries, the wizard Gandalf explained that this addiction transformed him into a miserable creature who talked to himself in two halves.
Saruman succumbed to pride and lust for power even though he never possessed the Ring. He sought to be powerful like Sauron and turned to evil through his own choices while refusing freedom when offered by Gandalf at Isengard.
The theme dates back to Plato's Republic where the Ring of Gyges gave invisibility and allowed its owner to get away with crime. Scholar Eric Katz notes Tolkien shows corruption through character thoughts and actions unlike Plato's abstract argument about moral life.
Samwise remains saved by love for Frodo and his unconquered plain hobbit-sense. He rejects using the ring because he understands where it leads and gives up the Ring willingly when needed most at Mount Doom.
George Orwell published Animal Farm from 1945 before William Golding released Lord of the Flies from 1954. Tom Shippey points out these authors dealt with corrupting influence contemporaneously with Tolkien during the years after the First World War.