Pretty much any time that Anderson shows any of the KKK members that he is not to be fucked with. The first time he basically vice-grips one guy's balls, squeezing hard enough that the guy ends up on the floor whimpering in pain (as Pell can only stare in horror (and this had been after he had been getting threatened by them), then beating the shit out of Pell in the barbershop, leaving him lying in a chair.
The FBI agents take Lester Cowans (shown to have the loosest lips in the Klan) for a ride around town, and lie to him about some of his fellow Klansmen accusing him of killing the three boys, when they know he only shot them after they were dead. Then they drop him off on the black side of town. Later than night, Lester's house is attacked by what he believes to be his fellow Klansmen. Just as he is about to be hung, the FBI Agents chase the Klan off. Then it turns out that the Klansmen were really undercover FBI agents, and they staged the attack so Lester would have no choice but to testify. It's pretty slick.