Count me in on Der Januskopf, too! After London After Midnight that's the one I'd like to see.

As to collecting every appearance by somebody I like, it is limited by what is available at the time. I think I may have everything that Chaplin did, but I'm not sure. I have all of the Keaton that is available for the silent period and even the box set of his Columbia shorts. I collect a lot of people by eras. While I am a big Bogart fan there are a couple of titles that were made in the 1950's that I don't have.

Now that you mention it, I'd probably have to catalogue the collection and then look through the imdb to see what I'm missing on some people and what is available.