Thanks. I actually wanted Totem to open any movie I clicked on from my file manager (Nautilus) in fullscreen mode by default. However, your suggestion gave me an idea and it's worked.
What I have done is look at the Properties of a typical movie (like a WMV for instance) and select the "Open with" tab, and "Added" another application to open all files of that type with. Rather than select an application from the list presented, I selected "use a custom command" and typed in "Totem --fullscreen". This allows the movie, and all movies of that file type, to be opened by double-clicking on its icon. However, because it is following the custom command, it will open them in Totem but with the full screen option switched on.
So, thanks for the idea of using the --fullscreen switch!