Candid photography is an absolute skill in its own right. You need to know your camera and settings inside out to react quickly to changing conditions. You need to be able to read a scenario and anticipate what will happen next and be ready to capture the moment and most importantly, you need to be creative and intuitive.
These are attributes of a photographer skilled in documentary images. Relying on program mode and machine gunning a wedding with your camera is not the answer.
You need to be comfortable in your surroundings and those around you must not feel intimidated by your camera. A long lens is, of course, a staple and if you're going to work hand-held, your shutter speed should be at least double the focal length you're shooting at. For example, 135mm on a 70-200mm lens, your minimum shutter speed should be no less than 250th of a second. If it's anything less, stick it on a monopod. Remember that shooting candids is all about NOT drawing attention to the camera so try and avoid flash if at all possible.
It's rather difficult to be in two places at once. You can't be capturing moments of spontaneity elsewhere when you're concentrating on what's in front of your own lens. So if you can, it pays to have a second photographer shooting in the opposite direction to your own camera. If you do, make sure they're taking photographs of something different. You don't want the same set of candids twice just taken from different angles.
As the wedding photographer, it's not just about pictures, it's about your people skills and making them feel comfortable within your space. Politeness, an engaging smile, how you present yourself and how you engage conversation will endear you to the people who surround you. Gaining a little trust and respect and projecting professionalism will blend you into the crowd as if you were a guest and it's then you can capture your images quietly and confidently, and go about your business hardly noticed.
Shooting at a distance, around f/4, is a great way to isolated people, especially if they are well lit against a plain background. Equally, though, shooting on an ultra wide at around 14mm and close can get you right in the heart of the action. Most people don't think they are in the frame when they see the lens isn't directly pointing at them so stay relaxed. But whatever you do, do not put the mother of the bride or groom at the edge of a wide angle shot, they will not thank you for it.
There are so many moments when you can capture a great candid but thinking along the lines of 'beauty' and 'moments of tenderness' are a good way to start. Capture when the dad sees his daughter for the first time in her wedding dress and focus on the bride's face as she pulls up outside the church or ceremony venue. Other key moments include the look of love on the bride's face as she looks at the groom, guests miles away with their own thoughts during the ceremony or the mass congratulations of the couple after the ceremony. Try to also look for themes within a wedding such as hats, shoes, socks, handbags, fascinators, colours and people holding hands that you could, perhaps, make a montage out of as well as use as standalone images.