Roy being so completelycalm during every action sequence, and never even raising his voice. His Dissonant Serenity alone probably contributes to at least some of June's hysterics.
During the warehouse shootout, Roy hands June a submachine gun for protection while he goes scouting. June waits nervously the whole time, and when he surprises her on his return, she screams for ten seconds straight, firing the gun the entire time in random directions and practically destroying the surrounding area. Two of the attacking mooks exchange glances with each other that scream "Seriously?" even through their balaclavas.