I've been sent a link (thanks Handsome) to a couple of videos on youtube which explains the Defiant gunner's duties and operations. Interestingly enough, the gunner controlled his turret by means of a joystick with a button on top for the guns. He also had a switch to make the turret motor move at a greater speed but was discouraged from doing so unless it was important. Also interestingly you'll see the switch which switches control of the guns between the gunner and pilot. Of course the pilot couldn't fire directly forwards as the rounds would take off the propeller (there was an interrupter gear to prevent this), the gunner had to elevate the guns to 19° or 20° to fire above the propeller arc. The pilot also did not have a reflector sight.
The section for the Defiant starts at 6:55 in this video and goes on for the next 2 videos after in the series, enjoy.