What is the best way to deal with a friend going through a bout of depression?

If the person is isolating him/herself, should you work extra hard to try to make plans, or be respectful of the desire to be left alone?

What about if the person is extremely cranky and irritable and hard to be around? Do you try to put up with it, or protect yourself and leave the person alone? What if the person is a coworker or someone you have to see every day not by choice?

Any stories, thoughts, etc appreciated.