The unwritten rule relating to the violence presumably committed by Lucas (a false presumption) calls for lynching - a public hanging without trial.
In his own way, Lucas defends himself through silence, recognizing the likely reaction to any public denial of the crime of which he stands accused.
This proud man of mixed blood, accused of murdering a white man, maintains his dignity by refusing to defend himself.
His silence is part of his protection. People do not know what to make of Lucas and his refusal to act according to expectations. Lucas protects himself through this act of silence and Gavin takes charge of speaking for Lucas and ensuring his safety against the mob.
Protecting the rights of African American citizens is not easy in Jefferson, but the good men of the town find a way to ensure that this is done. There is danger for Gavin in taking on the role of a protector for a man perceived to have crossed a distinct cultural line (killing a "white" man). Gavin understands his danger, but he recognizes that Lucas' danger is greater.
There is an honor and a dignity in the positions of Lucas and Gavin as they take sides against the injustice of a corrupt Southern code of knee-jerk retaliation. Gavin says to Chick at one point:
“Some things you must never stop refusing to bear. Injustice and outrage and dishonor and shame.”
Gavin and Lucas are not the only ones in danger. All of the African American prisoners are also in danger. If a lynching were to take place, which seems likely early on, the violence could easily get out of hand.
The danger in the air is recognized by all of Lucas' fellow prisoners and this is why they are so subdued.