I love Joel Cairo. The Maltese Falcon is one of my favorite movies and Joel Cairo is the funniest character. It is partially the way he looks but it's mostly his demeanor. The actor, Peter Lorre, is in a number of other big movies (Casablanca, The Man Who Knew Too Much) but his performance here is great. His character has a nondescript accent and a worldly way to him. I attribute it all to the directing. The movie is very melodramatic and Joel Cairo embodies the theme. Just go see the movie before I keep rambling.
p.s. It's also a very strong performance for Bogart. This movie's Bogeylicious.