You can display any operator in the viewer of a component. Most of the time you want to display the result or monitor a certain operator inside the component. If you want to show a TOP operator in the background of the viewer go to the Look page and point it to the Background TOP you wish to use.
If you want to display another not-TOP operator, you have to go to the Common page and give the Operator Viewer something to look at. For this to work Node View has to be set to Operator Viewer.