You make a function interactive by placing a list that begins with
the special form interactive immediately after the
documentation. A user can invoke an interactive function by typing
M-x and then the name of the function; or by typing the keys to
which it is bound, for example, by typing C-n for
next-line or C-x h for mark-whole-buffer.
Interestingly, when you call an interactive function interactively, the value returned is not automatically displayed in the echo area. This is because you often call an interactive function for its side effects, such as moving forward by a word or line, and not for the value returned. If the returned value were displayed in the echo area each time you typed a key, it would be very distracting.