create.js.erb

$('#ajax-modal').html(
  '<%= escape_javascript(
         render(:partial => 'watchers/new',
                :locals => {:watchables => @watchables, :users => @users})) %>');

<% if @watchables.size == 1 %>
  $('#watchers').html(
    '<%= escape_javascript(
           render(:partial => 'watchers/watchers',
                  :locals => {:watched => @watchables.first})) %>');
<% end %>