The two couples had been waiting to watch the new war film Lone Survivor at the Cobb Grove 16 cinema in Wesley Chapel when the row broke out.
Mr Reeves apparently stormed out of the auditorium to get a manager, but returned without one.
"Three seconds, four seconds later, the argument starts again," Mr Cummings said.
"Their voices start going up; there seems to be almost a confrontation. Somebody throws popcorn, I'm not sure who threw the popcorn, and, bang, he was shot."
Mr Oulson's wife, Nicole, was wounded as she had placed her hand over her husband just as he was shot, sheriff's spokesman Doug Tobin said.
A nurse in the audience tried performing emergency resuscitation on the victim while an off-duty sheriff's deputy detained the gunman, according to reports.
"The male victim is deceased. The female victim was injured with non-life-threatening injuries," Mr Tobin said.
Tampa Police spokeswoman Laura McElroy said Mr Reeves was a captain when he retired from the department in 1993, according to the Associated Press. After he retired, he was on the board of a neighbouring county's Crime Stoppers organisation.
The cinema, part of the Cobb Theatres chain, was reportedly evacuated and closed.
''It's crazy. I never thought something like this would happen at our theatre," ABC News quoted cinema employee Leny Vega as saying.
Lone Survivor, based on a New York Times best-seller, stars Mark Wahlberg and tells the story of four navy Seals on an ill-fated covert mission against the Taliban in Afghanistan.