redmine/app/views/watchers/create.js.erb

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

<% if @watchables.size == 1 %>
  <%= render(:partial => 'watchers/set_watcher',
             :locals => {:watched => @watchables, :user => User.current}) %>
<% end %>