Instructions
Be aware that to have a chance at getting a Cancer to open up to you, be gentle and pay close attention to how they feel. These individuals are very emotional creatures and only let certain people into their inner world. Ask them questions about their experiences and how certain things and situations make them feel. Cancers have a sense that they are difficult to understand and therefore greatly appreciate when someone puts in the effort to figure them out.
Know that Cancers are notoriously sensitive and for that reason even the most careful, tactful individual is bound to hurt their feelings at one point or another. Pay attention to insecurities that they may have mentioned and be sure to have the utmost sensitivity when these topics come up. Never try to joke about something them deem serious in order to make light of the situation--they will assume that you are mocking them and feel alienated by you. If you're not sure whether something you could say might hurt their feelings, it's better to say nothing at all.
Consider that at their best, Cancers are able to give an unending amount of support to their friends and loved ones. However, they do have a tendency to slip into periods of touchiness, moodiness and self-pity. There is usually something very specific that sets off such phases, but that doesn't mean they will necessarily clue you in to that reason. If you suspect something you have done has offended or hurt your Cancer friend, it's easy to make the mistake of assuming they just need some space (even if they're acting like they want space). Doing that will simply make them withdraw even more. Instead, force them to talk the issue out until you get to the bottom of it.
Know that if you're a casual friend of a Cancer, you might go through periods of time when you will not (and no one else will) hear from them. This is because Cancer is the sign of the home, and they have a tendency to go through phases of being a homebody (or even a hermit). This doesn't always mean that something is bothering them. Extensive social interaction can be really taxing for them and they need time alone in order to recharge. These are the situations when it's alright to give them some space. If you can't get a Cancer to come out, it's also smart to not show up on their doorstep. Their home is intensely personal to them and they're not the greatest hosts when they're not feeling social.
Consider that a Cancer is generally timid and shy, but they will go to great lengths to take care of and protect their loved ones. If anyone ever challenges or makes fun of your Cancer friend, be sure to do whatever you can to stand up for them. If you do not, they will feel betrayed and most likely withdraw from you for a time.