Huck is very realistic in his description of his ne'er do well father. He doesn't try to sugar-coat it; Pap Finn is no good:

Every time he got money he got drunk; and every time he got drunk he raised Cain around town: and every time he raised Cain he got jailed.

This is about as realistic a description of anyone you can get. Huck describes his father to a tee: an habitual drunk who's always getting into trouble with the law. He also provides us with a realistic glimpse into life in St. Petersburg and the values by which the townsfolk live.

Tom he made a sign to me—kind of a little noise with his mouth—and we went creeping away on our hands and knees. When we was ten foot off Tom whispered to me, and wanted to tie Jim to the tree for fun. But I said no; he might wake and make a disturbance, and then they’d find out I warn’t in. Then Tom said he hadn’t got candles enough, and he would slip in the kitchen and get some more. I didn’t want him to try. I said Jim might wake up and come. But Tom wanted to resk it; so we slid in there and got three candles, and Tom laid five cents on the table for pay. Then we got out, and I was in a sweat to get away; but nothing would do Tom but he must crawl to where Jim was, on his hands and knees, and play something on him.

In this next example of realism, we see Tom and Huck play a cruel trick on Jim. The scene is realistic because it illustrates the attitude towards slaves that was common among white people at that time, especially in the South. Though generally sympathetic characters, Tom and Huck are still a product of their time and place. They've imbibed the prevailing racial prejudices of the age and feel no shame in playing a cruel trick on someone regarded as less than fully human on account of his skin color.

Because he knew and loved the river, some of Twain's most descriptive and realistic writing is his description of the Mississippi River and the areas around it. He creates pictures of the scenes including every detail that would be noted by any character of the story who happened to be in the area along with Huck. It is easy for the reader to place him or herself in the setting.

I could see the sun out at one or two holes, but mostly it was bit trees all about, and gloomy in there amongst them. There was freckled places on the ground where the light sifted down through the leaves, and the freckled places swapped about a little, showing there was a little breeze up there.

Twain also took care when introducing new characters into the story. He provided enough detail to make it very clear and easy to understand the type of person and personality the new individual possessed. Huck provides information about Col. Grangerford's complexion, appearance, attire, mannerisms, and way of relating to those around him.