Generally when screen sharing you are connected to another system and are served whatever is being output to the monitor. In the sharing preferences you can select which accounts can use this feature, but there is nothing you can change about the overall functionality of the feature, which is to display the current contents of the screen to a remote location. Despite this, one user mentioned a small glitch that enabled him to essentially log in to the remote system in a separate account, and not see what was displayed on the remote screen.
Apple Discussion poster "denmoff" writes:
I've just upgraded two of my macs to snow leopard from leopard. I have two accounts, one is mine, the other is my wife's. My wife logged onto the iMac (the remote mac) with fast user switching turned on while my account was also logged on. When i used share screen from my macbook to my remote mac with my account, i got my account screen instead of hers. She continued to use her account with out any interruption from me. I could open apps and do as i please on the imac while my wife did her thing. This is not how it used to be with Leopard. If i used screen share, i would take control of her screen. My question is, is this supposed to happen? its kind of neat that it does this, but what if i want to control her screen? There isn't an option to change this."
This bug reminds me of similar situations I've seen where two separate functions have been melded together due to a glitch. For instance, in the game Quake 3 you could only start an internet server that ran in the background if you were not actually playing the game, or you could start a local network or "single-player" game. Despite these three options, I once played a game of Quake 3 where I started a local game to play with a few friends on the local network, but the game ended up broadcasting to the internet as a server for anyone to join. It was unsupported, but some glitch allowed it to happen.
Glitches such as that in my Quake 3 game and screen sharing expose the possibility for new functionality, which makes me wonder whether or not the programmers intended future support for the feature. For Screen Sharing, it would be exceptionally useful to log in from another system and choose either to view whatever is currently on the screen, or to see only your account privately but still allow others to use their accounts on the system. I imagine this would be exceptionally useful in a variety of scenarios; however, the closest that Apple has come to this so far is to have a "curtain" mode for Apple Remote Desktop. This locks out the local screen from anyone physically sitting in front of the computer so a screen sharing session can be private, but even this only allows for one physical user at a time. Support for multiple separate but simultaneous login sessions would be a boon to screen sharing.