What aspect of bill and sam’s behavior shows that they are round characters?

"what am i to do? " asks bill, looking at the kid suspiciously.
"you are the hoss," says black scout. "get down on your hands and knees. how can i ride to the stockade without a hoss? "
"you'd better keep him interested," said i, "till we get the scheme going. loosen up."
bill gets down on his all fours, and a look comes in his eye like a rabbit's when you catch it in a trap.
"how far is it to the stockade, kid? " he asks, in a husky manner of voice.
"ninety miles," says the black scout. "and you have to hump yourself to get there on time. whoa, now! "
the black scout jumps on bill's back and digs his heels in his side.
"for heaven's sake," says bill, "hurry back, sam, as soon as you can. i wish we hadn't made the ransom more than a thousand. say, you quit kicking me or i'll get up and warm you good."