Advicing the `interactive' form of a command

The following message is a courtesy copy of an article
that has been posted to gmane.emacs.help as well.

How can one advice the `interactive' form of a command with advice-add?
The elisp manual describes the different values for the WHERE argument
of `add-function' and `advice-add' and how they are equivalent to
lambda forms.  For example, :around advice is equivalent to

  (lambda (&rest r) (apply FUNCTION OLDFUN r))

This is clear for non-interactive functions.  But what happens
if I want to modify the interactive form of OLDFUN?

My slightly nontrivial example is `switch-to-buffer' where the
interactive form depends on `confirm-nonexistent-file-or-buffer'.
Indeed, the same var affects `switch-to-buffer' and `find-file'.
So I thought I could fine-tune things with something like

  (setq confirm-nonexistent-file-or-buffer nil) ; for `find-file'
  (advice-add
   'switch-to-buffer :around
   (lambda (fun &rest args)
     (let ((confirm-nonexistent-file-or-buffer t))
       (apply fun args))))

But this does not work.  It seems that the `interactive' form of the
advice'd command `switch-to-buffer' is run outside the let binding.
Actually, how does the lambda form in the above example work for a command
if the lambda form does not have an `interactive' form?  I looked into
nadvice.el, but something like `advice-eval-interactive-spec' is beyond
my understanding of things.