I find this an amusing question to answer, because in one sense, word choice simply refers to the words that the author choses to use in writing his novel. Thus any sentence from the novel would count as an example of word choice. However, we normally talk about word choice or diction to refer to the given effect that is produced by the careful selection of words for a specific purpose.
Let us consider the following example which I will then explain so you can see how we can talk about and analyse diction. This part of the novel is very key for a number of reasons, and comes at the end of Chapter Fifteen, after Huck has tricked Jim and Jim realises how he has been tricked:
It was fifteen minutes before I could work myself up to go and humble myself to a nigger; but I done it, and I warn't ever sorry for it afterwards, neither. I didn't do him no more mean tricks, and I wouldn't done that one if I'd a knowed it would make him feel that way.
Here we see a crucial stage in the moral development of Huck as a character. Note how the choice of words very simply, honestly and humbly admits his wrongdoing towards Jim and how he set about to make it right. The fact that it took "fifteen minutes" before he could apologise shows what a massive step this is for Huck, a white boy, to apologise to Jim, a black runawawy slave who is below him on the social ladder of the time. As if to emphasise that this was not just a passing fancy, he includes the phrase that "I warn't ever sorry for it afterwards" as if to confirm that this is a milestone in his development. The simple and direct choice of words here shows that this is an important stage in Huck beginning to realise that blacks are humans too.
In Mark Twain's The Adventures of Huckleberry Finn, word choice is especially important in order to convey the realistic tone of the South. Whereas a writer will make sure to use word choice in an especially careful way (using action verbs, avoiding slang and cliches), Twain is trying to present dialogue that sounds realistic.
Example of word choice in Huck Finn are found throughout the novel. In Chapter Thirty-Four, "We Cheer Up Jim," the conversation is as follows:
We stopped talking, and got to thinking. By and by Tom says:
'Looky her, Huck, what fools we are to not think of it before! I bet I know where Jim is.'
'In that hut down by the ash-hopper. Why, looky here. When we was at dinner, didn't you see a...man go in there with some vittles?'
'What did you think the vittles was for?'
'For a dog.'
'So 'd I. Well, it wasn't for a dog...It shows how a body can see and don't see at the same time.'
In the discussion, the Southern vernacular is necessary in order to provide a sense of the people who are speaking and the setting of the novel. It is also appropriate when the Duke and Dauphin try to pass themselves off as royalty. The language Twain employs sets the tone of the novel by creating more realistic characters.