public String intercept(ActionInvocation invocation) throws Exception 
{
        final Object action = invocation.getAction();
        final ActionContext context = invocation.getInvocationContext();
        if (action instanceof PortletRequestAware) {
            PortletRequest request = (PortletRequest) context.get(REQUEST);
            ((PortletRequestAware) action).setPortletRequest(request);
        }
        if (action instanceof PortletResponseAware) {
            PortletResponse response = (PortletResponse) context.get(RESPONSE);
            ((PortletResponseAware) action).setPortletResponse(response);
        }
        if (action instanceof PrincipalAware) {
            PortletRequest request = (PortletRequest) context.get(REQUEST);
            ((PrincipalAware) action).setPrincipalProxy(new PortletPrincipalProxy(request));
        }
        if (action instanceof PortletContextAware) {
            PortletContext portletContext = (PortletContext) context.get(STRUTS_PORTLET_CONTEXT);
            ((PortletContextAware) action).setPortletContext(portletContext);
        }
        if (action instanceof PortletConfigAware) {
        	PortletConfig portletConfig = (PortletConfig)context.get(PORTLET_CONFIG);
        	((PortletConfigAware) action).setPortletConfig(portletConfig);
        }
        if (action instanceof PortletPreferencesAware) {
        	PortletRequest request = (PortletRequest) context.get(REQUEST);
            // Check if running in a servlet environment
            if (request == null) {
                LOG.warn("This portlet preferences implementation should only be used during development");
                ((PortletPreferencesAware)action).setPortletPreferences(new ServletPortletPreferences(ActionContext.getContext().getSession()));
            } else {
            	((PortletPreferencesAware)action).setPortletPreferences(request.getPreferences());
            }
        }
        return invocation.invoke();
}

Sets action properties based on the interfaces an action implements. Things like application properties, parameters, session attributes, etc are set based on the implementing interface.