How much should you have in common with your partner?
For most people it is important you are not identical in your interests or temperment, but do have one or two good interests that you enjoy sharing.
If your mate likes to water-ski, go fishing, hiking, etc., then try finding something in there that you like and do it together. This does not mean you have to do these things constantly whenever he/she may want to go hiking (that's what friends are for.)
Most people like to be intellectually stimulated. They enjoy talking over matters of the world with each other. That's healthy! Pick at least one thing you both truly enjoy doing and that should be enough. Spending time together (perhaps one day on the weekend) just having fun or doing what you like to do best is all it takes. Give each other some breathing room. Never shut your friends out and it's also healthy to be independent from each other. You could enjoy classes on digital graphics, and even take a course with a friend, and he/she may enjoy going out with friends or being active in some sport. Being apart from each other once in awhile is good.
If one is of a quiet nature and you may be extroverted accept this because the two go together well. If you are both extroverted that's great too.
Hope I have answered your question and it helps.
Usually as much as possible. This excludes traits such as selfishness, dishonesty and jealousy, of course, since there should be as little of those as possible.