How can I activate interactivity on cartoDB.js with createLayer?

Question

I've made a map with cartoDB.js with the createLayer method. I've defined the sublayers,inside a variables called layerSource, giving them an interactivity field like this:

sublayers:[ {sql: "SELECT * FROM db_name",
interactivity: "cartodb_id",
cartocss: "styles"},...more sublayers...

After this I've added the layers to the map

cartodb.createLayer(map,layerSource)
                    .addTo(map)
                    .done(function(layer) {
                       for (var i = 0; i < layer.getSubLayerCount(); i++) {
                           sublayers[i] = layer.getSubLayer(i);
                       }
                    })
                    .error(function(err) {
                        console.log("error: " + err);
                    });

But the sublayers are not interactive. I've seen in the documentation that I have to enable the interactivity with setInteraction:true, but where I should activate this option?? Inside the createLayer (as an argument?), inside the .done??.. I assume this should be something very obvious but I'm new to javaScript and not being able to get this done.

EDIT:

After @Andy_Eschbacher answer.
I'm trying to do what you said but with the first block of code

.done(function(layer) {
                      for (var i = 0; i < layer.getSubLayerCount(); i++) {
                           sublayers[i] = layer.getSubLayer(i);
                       };

                        sublayer[0].on('featureClick',function(event, latlng, pos, data, layerIndex) {
                            console.log(data);
                        });
                    })
                    .error(function(err) {
                        console.log("error: " + err);
                    });

the console sends an error because sublayer is not defined, so I define a sublayer = layer.getSubLayer(0);as you do in this tutorial but no action is triggered.

Answer 1

You enabled interactivity, but you've not created infowindows, hovers, or printed anything to the console yet.

If you add the following within .done(function(layer) {....});, it will print to the console:

sublayers[0].setInteraction(true);
sublayers[0].on('featureClick',function(event, latlng, pos, data, layerIndex) {
    console.log(data);
});

If you want to enable infowindows, you need to add the following:

cdb.vis.Vis.addInfowindow(map, layer.getSubLayer(0), ['cartodb_id']);

This process is walked through in a tutorial on making public maps from private data.