Twain certainly has a flexible style; he can suit his words quite easily to the situation, whether he is describing the thoughts of Tom or expounding upon some lofty subject. That said, one of the most impressive aspects of his writing is the dialogue. Rather than try to describe it, we'll give you a taste. Tom, having just barely survived an encounter with Injun Joe the night before, goes to talk with Huck:
[Silence, for a minute.]
"Tom, if we'd 'a' left the blame tools at the dead tree, we'd 'a' got the money. Oh, ain't it awful!"
"'Tain't a dream, then, 'tain't a dream! Somehow I most wish it was. Dog'd if I don't, Huck." (27.4-8)
Twain renders the rhythm of the speech beautifully, using contractions and slang to give life to Tom and Huck's speech. Small touches, like the "we'd 'a'" and "dog'd" go along way toward making them sound like real boys. Even that bracketed pause speaks volumes. Without suitable dialogue, all of Twain's observations and remarks would be like a cake without icing: still delicious, but not quite right.