how to enable interactivity on named map in cartodb

I have created a named map that gets data from a private table. I can load it into a Leaflet map and can change its marker-fill color placeholder using layer.setParams.

However, I cannot get interactivity to work. I want to be able to hover or click on a feature and do something useful.[1]

What do I need to do to make hover or click events fire?

Here is the named map json file

{
  "version": "0.0.1",
  "name": "mynamedmap",
  "auth": {
   "method": "open" 
  },
  "placeholders": {
      "color": {
        "type":"css_color",
        "default":"blue"
      }
  },
  "layergroup": {
    "version": "1.0.1",
    "layers": [{
      "type": "cartodb",
      "options": {
        "cartocss_version": "2.1.1", 
        "cartocss": "#layer { marker-fill: <%= color %>; }",
        "sql": "select * from mytable",
        "interactivity": ["cartodb_id"]
      }
    }]
  } 
}

Here is the javascript to create the layer:

cartodb.createLayer(map, {
        type: 'namedmap',
        user_name: 'myusername',
        options: {
          named_map: {
            name: 'myusername@mynamedmap'
          }
}})            
.addTo(map)
.done(function(layer) {
    console.log(layer.getSubLayerCount()); //prints '0'        
    layer.setParams('color', 'green'); // works to change marker color

    layer.on('featureClick', function(e, latlng, pos, data) {
      console.log(e, latlng, pos, data); //this never gets called
    });

  });

[1] "Something useful" could be as simple as open infowindow and display cartodb_id and feature name. Or it could mean "get cartodb_id for feature, pass it to my proxy where I can use curl to run some custom sql against my private table, then return json to the map and update display"