Sam and Bill share many similar traits because they have similar histories. What they do for a living, the absurd and humorous situations they both get themselves into because both of them have the same goals: money; they're both disillusioned about how things work, and both ultimately end up in a bad situation.
First, their history. Both characters have criminal pasts: their attempt to kidnap a respected member of Summit, Alabama's son, in order to receive ransom money (all a criminal act) to pay off a prior criminal act (some fraudulent scheme) is one example.
Secondly, although neither Sam nor Bill find their situation to be humorous in any way, they are both portrayed as comical half-wits because they actually think that because the town is so small and has little in the way of a police force, that the kidnapping should work without a hitch. Additionally, they haven't thought through what they are going to do with the boy once they kidnap him, as neither of them have planned that far in advance, or really know how to take care of a child. Instead of the kidnappers terrorizing the boy, the boy is the one terrorizing Sam and Bill.
Thirdly, Sam and Bill try to appear as though they are intelligent through their use of sophisticated language and ability to write and retell the story of their harrowing experience--except that they don't use it properly—and by thinking that the handling of the ransom note—with their non-negotiable demands—is a done deal, but then they sign it as "two desperate men." Again, because they think they're both a lot smarter than they actually are, and didn't plan the kidnapping ahead of time, not only do Sam and Bill not make the $2500 they needed, but they end up losing the $600 that they started with. The fact that Sam and Bill felt the need to retell their absurd story—to the pure enjoyment over readers everywhere—shows that they have absolutely no idea what they’re doing, why they’re doing it, or the consequences caused by their actions.
Consequently, all they managed to do for themselves is let everyone they’ll come in contact with know their not so well-kept secret: they’re really just a pair of cowardly, harmless, amateurs who are more likely to give themselves away than do any harm.
While there are differences between Bill and Sam, there are many similarities. They are both criminals, or at least trying to be; neither has the slightest propensity to violence, neither has a clue what to do with a child, and both have a pretentious and comical way of expressing themselves.
First, both Bill and Sam, the narrator, are criminals. They hatch a plot to kidnap a child to raise more capital, "to pull off a fraudulent town-lot scheme in Western Illinois with" (para. 3).
Second, while both Bill and Sam are criminals, by the end of the story it is clear that neither of them has a capacity for violence. There is no evidence that any of their criminal plans or endeavors has led to hurting anyone physically, and they cannot bring themselves to hurt "Red Chief" in any way, even as he hurts or threatens to hurt them.
Third, Bill and Sam's handling of Red Chief makes clear that neither of them knows how to deal with a lively and mischievous child. By the end of the story, Red Chief has terrorized both of them, so badly that they actually pay to return him to his father.
Fourth, both Bill and Sam have a way of slaughtering the English language, trying to sound well-spoken. For example, as Sam notes, Bill said that they formulated their plan "during a moment of temporary mental apparition" (para. 1). The word that Bill had in mind was clearly "aberration." Sam has this tendency as well, to use big words incorrectly, saying,
It's an awful thing to hear a strong, desperate, fat man scream incontinently in a cave at daybreak (para. 22).
All in all, these similarities contribute to the humor of the story: two pretentious and bumbling but harmless criminals who are clueless about what to do with the kidnapped child who defeats them.