How do I activate jQuery/script on demand?

1. Register jQuery and the plugin within your head

wp_register_script( 'jquery' );
wp_register_script( 'your_jquery_plugin', STYLESHEETPATH, 'jquery', '0.0', false );

2. Register a meta box

It should be a simple checkbox. Please do it like it’s described on the codex page for add_meta_box() (don’t just repeat this one here). If you follow the rest, then the $id for the meta box should be 'jquery_comments'.

Example (shortened):
inside functions.php

    function add_jquery_comments_meta_box() {
       $id = 'jquery_comments';
       $title = __( 'jQuery Comments, please?', 'your_textdomain_string' );
       $context="side"; // advanced/normal also possible
       $priority = 'low'; // high also possible
       $callback_args=""; // in case you want to extend it
       add_meta_box( $id, $title, 'add_jquery_comments_cb_fn', 'post', $context, $priority, $callback_args );
    }
    add_action( 'admin_init', 'add_jquery_comments_meta_box', 1 );

    // Prints the box content
    // Please adjust this to your needs - only slightly modified from codex example
    function add_jquery_comments_cb_fn() {
       // Use nonce for verification
      wp_nonce_field( basename(__FILE__), 'your_noncename' );

      // The actual fields for data entry
?>
      <label for="jquery_comments"><?php _e("Do you want jquery comments on this post?", 'your_textdomain_string' ); ?></label>
      <input type="checkbox" id="jquery_comments" name="jquery_comments" />
<?php
    }

3. enqueue the script based on your meta box as the condition

and then write a function like this in your functions.php file

function add_jquery_comments_plugin() {
   wp_enqueue_script( 'jquery' );
   wp_enqueue_script( 'your_jquery_plugin' );
}
// and call it depending on a conditional in the comment form hook
if ( get_post_meta($post->ID, 'jquery_comments', true) ) {
   add_action( 'comment_form', 'add_jquery_comments_plugin' );
}