Two couples were watching Lone Survivor at The Grove 16 cinema in Wesley Chapel, north of Tampa, when 71-year-old Curtis Reeves asked Chad Oulson, 43, to stop texting. Police said the argument that followed led to the shooting.
Reeves has been charged with second-degree murder.
Mr Oulson and his wife Nichole, who was also shot and injured, were taken to hospital where Mr Oulson later died.
An off-duty police officer detained Reeves until colleagues arrived.
Charles Cummings, at the cinema for his birthday with his adult son, said a man in the back row got up and left the auditorium, presumably to get a manager. But he came back alone after a few minutes, appearing upset. Moments later, the argument between the two men resumed and they started raising their voices.
"Somebody throws popcorn. I'm not sure who threw the popcorn," Mr Cummings said. "And then bang, he was shot."
Mr Cummings said Mr Oulson fell on to him and his son and his son went to call 911, while he and another patron who claimed to be a nurse began performing CPR on Mr Oulson.
A man sitting next to the gunman grabbed the weapon out of his hand and the suspect did not attempt to get away, Mr Cummings said.
"I can't believe people would bring a pistol, a gun, to a movie," he said. "I can't believe they would argue and fight and shoot one another over popcorn. Over a cell phone."