MINNEAPOLIS — He chatted with a store clerk about playing football. He grabbed a banana off a shelf, flipped through a wad of cash, and hugged and exchanged pleasantries with a woman, laughing with his hand on her back.

In surveillance footage played for the first time in a Minneapolis courtroom on Wednesday, the world got to see George Floyd as it never had before: He was just another customer in a corner store that he liked to frequent.

Within half an hour, Mr. Floyd would be handcuffed and face down on the pavement outside of Cup Foods, calling out for his mother as a police officer pressed his knee into Mr. Floyd’s neck. Roughly two hours after he walked into the store he was dead.

On the third day of testimony in the trial of Derek Chauvin, the former police officer charged with murdering Mr. Floyd, a clearer picture emerged of the events preceding Mr. Floyd’s death, with witness after witness agonizing over whether they could have done anything to stop what would soon unfold.

The 19-year-old clerk who served Mr. Floyd at the corner store that day wondered whether the death was his fault because he had reported that Mr. Floyd used a fake $20 bill. A 61-year-old man who saw the police pinning Mr. Floyd to the ground shook his head and held back tears as a video of the brutal arrest played. He collapsed on the witness stand, sobbing. “I can’t help but feel helpless,” said the man, Charles McMillian. “I don’t have a mama either, but I understand him.”