Not only can you have a valid OOP program in which you have never
declared an interface, you can have a valid OOP program in which you
never even _use_ an interface.
Which is not to say that I can say for sure your interface-less program
is a good OOP program. But it's possible.
As for how you can "get more intuitive about creating interfaces", I
don't really know how to answer that in a specific way.
In general, you "get more intuitive" about all programming topics by
practicing, a _lot_. The more you write C# programs, the more you will
find yourself in situations where it makes sense to declare some
particular common functionality as a type for polymorphic behavior, but
where there is no default implementation per se, and where the
functionality doesn't fit into the "is a" mold (i.e. the kinds of
situations where one would be inclined to declare an interface).
And of course, the more you find yourself in such situations and
practice handling them, the more "intuitive" you will "get" regarding
such situations.
Pete
