The physics of heavy quarkonium suppression in quark-gluon plasma has been discussed in the context of screened QCD force in medium for a long time. In this talk, I will show that another type of force, namely the drag force and its fluctuation, also plays an important role in the real-time dynamics of heavy quarkonium in the medium. In order to make this statement concrete, I will employ the closed-time path formalism of non-equilibrium quantum field theory and perform leading order perturbative analysis. This will tell us how these different types of forces determine the dynamics of quantum states of heavy quark systems in the medium.