You need a poker face for taking secretive pictures, particularly when the people are sitting opposite you. I'm currently reading Paul Auster's "The Music of Chance" which is about poker players and gamblers (well it is so far - please don't let me know how it ends if you've already read it - or watched the film). I love Paul Auster, he writes in such an effortless conversational way ("I've figured these mugs out, I'm so on top of it I can tell what cards they're holding just by looking at their eyes") and yet there's substance and form and thought to it.
Tobacco was a useful prop, and it helped to prevent his opponents from watching him too closely, as if he could literally hide his thoughts behind a cloud of smoke. The important thing is to remain inscrutable, to build a wall around yourself and not let anyone in.
Sound advice for me as a "commuting spy" (although not with all the no smoking signs around) but surely building a wall around one's self is what most people do on the tube, so no wonder I can spy fairly easily. Perhaps I should take up poker too.