Wordpress - How to prefill the username/password fields on the login page

The Result

Username

You can declare the form field value global and prefill it.

This will override any setting done via the "Remember me"-Checkbox.

/**
 * Changes the default user name to "DEMO"
 * 
 * @return string $user_login
 */
function login_form_username()
{
    global $user_login;
    return $user_login = 'DEMO';
}
add_action( 'login_head', 'login_form_username' );

Password

I found nothing that you can use to prefill it, so I'd add a note. Gladly/Sadly there's the whole set of stylesheets for the admin UI available. So .wrap in connection with h2 gives a nice styling. The function is hooked to the additional fields hook for the admin hook. The priority is 0 to set it above additional fields.

/**
 * Adds a note beyond the user login name & password field
 * 
 * @return string
 */
function login_form_note()
{
    print '<div class="wrap"><h2 style="text-align: center;">The Password is "DEMO"</h2></div>';
}
add_action( 'login_form', 'login_form_note', 0 );

Security: Hide Errors

As someone would get a visual proof that he entered an existing user name by default, I'd add the following to your functions.php file, to avoid telling if the username was guessed right:

/**
 * Hide wrong login names
 * 
 * @return string
 */
function no_login_error() 
{
    return __( 'Wrong Credentials.' );
}
add_filter( 'login_errors', 'no_login_error' );

Other filters

Notes:

• If you want to replace other stuff like styles, then you can use the login_enqueue_scripts hook.
• You can also replace the link behind the logo using the login_headerurl filter that filters the url. The link title can be replaced using login_headertitle. Both trigger for multisite as well a single site setups.
• The login message can be changed using the login_message filter.

you could use this code to auto login the user:

add_action('init', 'auto_login');
add_action('admin_init', 'auto_login');
function auto_login() {
    if (!is_user_logged_in()) {
      //by user name
      $user = get_user_by( 'login', 'demo' );
      //Or by user id, 2 being the ID of the demo user
      //$user = get_userdata(2);
      wp_set_current_user($user->ID, $user->user_login);
      wp_set_auth_cookie($user->ID);
      do_action('wp_login', $user->user_login);
    }
}

Here are 3 ways to do it:

The first way is to use the wp_signon hook

$creds = array();    
$creds['user_login'] = 'example';   
$creds['user_password'] = 'plaintextpw';   
$creds['remember'] = true;   
$user = wp_signon( $creds, false );  
if ( is_wp_error($user) ) {  
   echo $user->get_error_message();
}

The second way, is to use jQuery .val() and add it to your login page

$("#user_login").val('username');    
$("#user_pass").val('password');

The last way is to edit your wp-login.php file (Note, this is not recommended although it would work).