Greenstreet and Lorre had never appeared together in a film prior to "The Maltese Falcon", although they were to appear in a subsequent five (all excellent) films together,
because "The Maltese Falcon" was Sydney Greenstreet's very first film.
That's interesting. And the trouble with making a debut in a film like that is the risk of type casting, I suppose. I can think of three others where they appeared together: Casablanca, Background to Danger and Mask of Dimitrios. Can't think of the other two right now.