Quote Originally Posted by alexei90 View Post
I don't mean to hi-jack the thread, but could you explain that? I always thought they were interchangeable and meant "someone," whereas кто-либо meant "anyone."
No, кто-либо is just another way to say кто-нибудь. Just like there is another way to say кто-то, which is кое-кто. The last two can have slightly different meanings sometimes, not that important though. Anyway, the difference between кто-то and кто-нибудь is just like what you have described earlier:

кто-то when you have a specific person in mind, even though you might not know their identity.
кто-нибудь is when you don't have any clue or jsut don't care who that who might be, anyone will do

For more indepth explanation you can search this forum, there were many threads about these words, you can find more of my posts there even