The minibuffer prompt displayed by functions that use (interactive) is
usually
highlighted with the face `minibuffer-prompt'. However, interactive specs
"c",
"k" and "K" don't do this.

To reproduce:

  emacs -Q
  M-z

Running this function shows the difference more clearly:

  (defun example-prompt (&rest args)
    (interactive "sXXXX: \ncXXXX: \nkXXXX: \nKXXXX: ")
    (ignore args))

In GNU Emacs 23.0.60.1 (i486-pc-linux-gnu, GTK+ Version 2.12.11)
 of 2008-11-22 on elegiac, modified by Debian
 (emacs-snapshot package, version 1:20081121-1)
Windowing system distributor `The X.Org Foundation', version 11.0.10402000
configured using `configure  '--build' 'i486-linux-gnu' '--host'
'i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib'
'--libexecdir=/usr/lib' '--localstatedir=/var' '--infodir=/usr/share/info'
'--mandir=/usr/share/man' '--with-pop=yes'
'--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/23.0.60/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.0.60/site-lisp:/usr/share/emacs/site-lisp'
'--with-x=yes' '--with-x-toolkit=gtk' 'build_alias=i486-linux-gnu'
'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -DSITELOAD_PURESIZE_EXTRA=5000
-g -O2' 'LDFLAGS=-g -Wl,--as-needed' 'CPPFLAGS=''

Also observed in GNU Emacs 22.2.1 (i486-pc-linux-gnu, X toolkit, Xaw3d
scroll bars) of 2008-11-10 on raven, modified by Debian