For the Standard Model extended with a real scalar singlet field, the modification of the heavy Higgs signal due to interference with the continuum background and the off-shell light Higgs contribution is studied for gg→ZZ,WW→4 lepton processes at the Large Hadron Collider. Interference effects can range from O(10%) to O(1) effects for integrated cross sections. Despite a strong cancellation between the heavy Higgs–continuum and the heavy Higgs–light Higgs interference, the full interference is clearly non-negligible and modifies the heavy Higgs line shape. A |MVV−Mh2|<Γh2 cut mitigates interference effects to O(10%) or less. A public program that allows one to simulate the full interference is presented.