You can set Screens in Observe or Control mode:
ObserveThis mode lets you see what is happening on the remote computer but will disable the Shortcuts Toolbar, Keyboard and will not send any mouse commands. You can also set Screens to use this mode automatically in the screen settings.
ControlThis is the default mode. Keyboard and mouse commands are sent to the remote computer along with any shortcuts from the Shortcuts Toolbar.
To select a mode while you are connected, tap on the Observe/Control button in the toolbar.