I was recently trying to pin down exactly what kinds of tactics I was employing when grabbing candids mainly at wedding receptions. Here are a few that I find that I use.
1. Engaged people are easier to photograph in candid fashion. Seek out pockets where people are focused on and talking with one another. Make good use of the time for which attractions (speeches, games) are garnering the attention of guests. To this end, encourage the B+G to make the rounds to each of the tables between courses.
2. Some guests are super paranoid and vigilant about spotting the camera (they then clam up !). If you've made eye contact with the subject you want to be shooting, pivot to act as if you're shooting another table. As you lift your eye from the camera, check back to see if the intended subject is relaxed again and not focusing on your presence. Grab the shot.
3. Try to stalk around with the camera down at your waist level as much as possible, reserving raising the camera up to your eye only when you're ready to grab the shot. Try to do most of your seeing not through the prism.
4. Shoot pre-shots from the hip. If you move to another part of the room with different lighting (or variables are changing with respect to your flash), take a test shot from the hip of, e.g. the table you're considering shooting. Then if needed, make adjustments. Then shoot the table. If you approach the table, shoot, realize you have to make an adjustment, it often happens that as a photographer you're exposed and lose the possibility for candid shooting at that table--at least for a while.
5. Watch the action and anticipate. See that girl laughing uproariously at that dude's joke ? You missed it the first time, but bank on it happening again. Set up for her and just wait it out for at least a few seconds before moving on. Remember that one great photo every minute is a fairly good success rate. Sometimes anticipating well means focusing on the right guest. If the bride is giving her speech, go find and focus on her parents for a few minutes.
6. I tend to prefer shooting candids at a shorter focal length (50mm on a full frame); but for the tougher tables / areas, I'll shoot long.
7. Especially if you're shooting with a fast lens, see if you can get away with not using focus-assist. Take some shots, assess your success rate at grabbing focus in that light.
8. Encourage moody lighting. Not only do guests generally prefer to party in an environment not resembling a supermarket, but it makes it far far easier to be sneaky.
9. If you have an assistant with you who doesn't need to be schlepping at that very moment, mill around talking to them as if you both are guests. While you're talking, watch for nearby subjects, raise the camera quickly, grab the shot, then focus on your assistant again. This tactic is ideal for the stand-up cocktail portion of the day.
10. If you do get caught by your subject just as you're grabbing a photo of them, keep focus, hold the camera in place as you raise your head, engage with them, be affable, and take the photo at a moment of maximum expression (recall: you're still not looking through the prism). People often don't think you're actually photographing UNLESS your camera is to your eye. Alternatively, if you're dealing with the mugging sort or a couple who actually wants to be shot. oblige them.
Feel free to add your own tactics to what is definitely not an exhaustive list.


