Redirect to login page, or show login page instead of authenticated content?

[sKunKbad]

I was just wondering what people think, and what are your reasons why redirecting to a login page, or showing the login page instead of authenticated content is the right way.

 

To be clear, lets say that a site visitor requests a page that requires authentication, and that site visitor is not logged in.

 

1) Should they be redirected to a login page?

 

2) Should the login page magically appear without redirect, replacing the content that would have been showed if they were logged in?

 

Does it really matter which way login is handled? I have not been using redirects, and not experienced any problems with showing the login page instead of the authenticated content. It's actually very convenient to do login this way (at least for me). Are there any issues to be concerned with?