http://codex.wordpress.org/Function_Reference/wp_list_comments

remove the callback code from the wp_list_comments() code in comments.php:

change this line:

<?php wp_list_comments( array( 'callback' => 'twentytwelve_comment', 'style' => 'ol' ) ); ?>

to:

<?php wp_list_comments( array( 'style' => 'ol' ) ); ?>

or alternatively, also copy the function twentytwelve_comment() from Twenty Twelve’s functions.php, and add it into your theme’s functions.php; code below:

/**
 * Template for comments and pingbacks.
 *
 * To override this walker in a child theme without modifying the comments template
 * simply create your own twentytwelve_comment(), and that function will be used instead.
 *
 * Used as a callback by wp_list_comments() for displaying the comments.
 *
 * @since Twenty Twelve 1.0
 */
function twentytwelve_comment( $comment, $args, $depth ) {
	$GLOBALS['comment'] = $comment;
	switch ( $comment->comment_type ) :
		case 'pingback' :
		case 'trackback' :
		// Display trackbacks differently than normal comments.
	?>
	<li <?php comment_class(); ?> id="comment-<?php comment_ID(); ?>">
		<p><?php _e( 'Pingback:', 'twentytwelve' ); ?> <?php comment_author_link(); ?> <?php edit_comment_link( __( '(Edit)', 'twentytwelve' ), '<span class="edit-link">', '</span>' ); ?></p>
	<?php
			break;
		default :
		// Proceed with normal comments.
		global $post;
	?>
	<li <?php comment_class(); ?> id="li-comment-<?php comment_ID(); ?>">
		<article id="comment-<?php comment_ID(); ?>" class="comment">
			<header class="comment-meta comment-author vcard">
				<?php
					echo get_avatar( $comment, 44 );
					printf( '<cite class="fn">%1$s %2$s</cite>',
						get_comment_author_link(),
						// If current post author is also comment author, make it known visually.
						( $comment->user_id === $post->post_author ) ? '<span> ' . __( 'Post author', 'twentytwelve' ) . '</span>' : ''
					);
					printf( '<a href="%1$s"><time datetime="%2$s">%3$s</time></a>',
						esc_url( get_comment_link( $comment->comment_ID ) ),
						get_comment_time( 'c' ),
						/* translators: 1: date, 2: time */
						sprintf( __( '%1$s at %2$s', 'twentytwelve' ), get_comment_date(), get_comment_time() )
					);
				?>
			</header><!-- .comment-meta -->

			<?php if ( '0' == $comment->comment_approved ) : ?>
				<p class="comment-awaiting-moderation"><?php _e( 'Your comment is awaiting moderation.', 'twentytwelve' ); ?></p>
			<?php endif; ?>

			<section class="comment-content comment">
				<?php comment_text(); ?>
				<?php edit_comment_link( __( 'Edit', 'twentytwelve' ), '<p class="edit-link">', '</p>' ); ?>
			</section><!-- .comment-content -->

			<div class="reply">
				<?php comment_reply_link( array_merge( $args, array( 'reply_text' => __( 'Reply', 'twentytwelve' ), 'after' => ' <span>&darr;</span>', 'depth' => $depth, 'max_depth' => $args['max_depth'] ) ) ); ?>
			</div><!-- .reply -->
		</article><!-- #comment-## -->
	<?php
		break;
	endswitch; // end comment_type check
}