The interminable trial of Jeffery Williams — Young Thug — in Atlanta has again veered off the highway, after a high-profile witness at the center of the case was arrested on the stand after refusing to testify.

Kenneth “Lil’ Woody” Copeland, a Ysl associate whose leaked interrogation tape last year left him labeled as a snitch, has long been anticipated as a witness for the state, but his appearance in court Friday came with only a day’s public notice. Prosecutors previously said he had been threatened as a high-profile witness.

According...