You can display various shaped elements, such as an Interface, Use Case or Actor, using rectangle notation. This displays the element as a rectangle, with an icon of the 'normal' shape in the top right-hand corner. Any attributes, operations or constraints belonging to the element are shown, in the same style as a Class.
To show an element using rectangle notation, right-click on the element on the diagram and select the 'Advanced | Use Rectangle Notation' context menu option. This setting only applies to the selected element, and can be toggled on and off either by deselecting the context menu option or by selecting the reciprocal option such as 'Use Circle Notation' or 'Use Actor Notation'.