If you shake the can then it wont only fizz and sizzle but explode because the citric acid will make the bubbles with the other ingredients will make CO2 and the solution will have no where to go but...
Because of the difference in the volume of dissolved solute (sugar/aspartame), Diet Coke tends to be "fizzier" than regular Coke (it will release its dissolved CO2 more easily in sresponse to heating...