No, they are not interchangeable. If you "tell" somebody to do something, you are commanding them to do it. Generally, it's rude to do this unless you are in a position of authority. If you "ask" them, it is a request. It's also much more polite, so even people in authority will ask most of the time because they want people to do things of their own volition rather than because they have been commanded to do so. The exception would be if they need immediate action. If I tell my wife: "Go get me a cup of water!" (telling), the response would be much different than if I say, "Can you please get me a cup of water?" (asking)