who the hell cares what lens use for the snapping expression. just dont forget the focus and zoom (if the zoom is your concern, just move into or away from subject). CONCENTRATE on the subject. forget the lense.
I dont care what lense it takes. if i bring along 85mm so be it, i will just move back a little. it was 300mm, then i go to move in closer. I dont know why you (the tread starter) have this worrying feeling - just let yourself go and capture the moment using what you have. WHO IN THE WORLD CARES IF YOU USE a mere 24mm lense when the moment of expression was not captured ?
Integrasky - Think about it - "where I stand too far ..." - are you uncomfortable taking the shot in the first place?! If so, you're not even focusing on your subject, but on yourself, and ANY lens you use would yield less than "interesting" results. Also, when you're sort of "across the street" with a long thing sticking out the front of your DSLR, you will certainly attract way more attention as compared with the casual passerby who snaps with a less threatening, "less professional" setup!
As pointed out above, if posed, anything goes, but if you intend taking candids at a "reasonable" perspective, then for head or upper-torso, you'll get along just fine with any of an 85mm, 90mm or 100mm on a typical DSLR (not full-frame). More important it is to not feel self-conscious, to learn how to feel and look natural with your camera, and to respect your subjects! Why, with the very young and the very old, you can get superb shots even with a 30mm lens from across the dinner table! (Not advisable for anybody else not in the above groups, though!!!)
A warning, though - you might actually have more success with your kit lens prefocussed to the intended distance (appropriate aperture, of course), rather than the higher-image-quality lenses in the ranges described above, as poorly focussed shots with these lenses stick out unbelievably obviously!!! We've all done it, unfortunately, and it only reduces with tons of practice and good judgment of distance. The good prime lenses highlight every flaw in equipment AND technique!