When I switch to another controller do I really have to rebuild every part on the page?

[eluser]Scouser[/eluser]
What I mean is, I have a page, build up of several partial page 'blocks', with an optional link to "login" which is in fact a different controller. But when I display the login page I still need some (or all) of the page 'blocks' or elements surrounding the login page.

The question is, when I switch to another controller do I really have to rebuild every part on the page or can I just replace the (for example) body part of the page with this new part called "login page"?