I had cancer when I was younger, and the best place to start is to ask yourself: "Do I need to say anything at all?"

Unless you're a close friend (with whom the patient is completely at ease), then you're putting them in a position where they feel like they have to act strong, smile, and thank you for your concern. (For my situation, I got dozens of phone calls from people I hadn't heard from much in years. I had more than enough on my mind than to appease their desire to show their concern.)

Your concern is definitely appreciated, but you need to choose your moments.

As for what to say? I guess that differs culturally and your relationship. I'm Australian, and I was totally okay with a simple "ah, shit mate, that sucks" over a "if you need ANYTHING, call me* (*but don't actually call me)" - I know I can call for help, and dozens of people can help, but all I really wanted was to be treated like a normal human. For instance, I was bald from the chemo and got a multi-colour clown wig as a joke present, which made me smile so much. It was a sense of "everything is normal despite being in the midst of chaos." I could count on my friends to distract me from the horrible reality of the situation when I was stressed/sad/confused.

And that is your job. Be a sincere option for distraction and advice when you're called upon. Trust them to ask for help when they need it..and they're much more likely to ask for it if they know you won't be overbearing/over-worried/judgemental/etc.

I don't know how well this advice applies to depression, but I think it's a good framework for being a good friend through most hardship.

(Sidebar: I'm more than happy to be a sounding board for approaching any cancer related issues you've got. Email's in my profile. I've heard all kinds of things, so don't think any question or issue is too simple or stupid!)

Thanks very much, your response is helpful. Thanks for the offer, too. Thankfully, I'm not struggling with this personally now, but I'm sure there are people here who are.

You don't have to say much. For example, my wife will ask 'is there anything I can do?' when I'm having a particularly bad time, and while the answer is often just 'no,' her asking the question that way acknowledges the fact that I'm struggling without putting me under pressure to come up with an answer. For me the most important thing is knowing that someone is available without feeling that they're waiting on me.

Chronic depression is a bit like an old AM radio; you can mitigate the lousy signal with the tuning knob and moving the antenna around, but you can't fix it and call it done; the station that's clearly tuned in today might stay clear tomorrow or be mired in static. As a patient, one can learn to separate oneself from one's mood, so that feeling miserable doesn't have to be attributed to an objective exterior cause. But learning that takes time, and inability to distinguish between one's emotional and the quality of one's environment was a major problem for me when I was younger.

The most helpful thing you can do is be around to listen and allow the depressed person to vent without pushing them to identify an actionable solution, frustrating as that may be. Asking questions can also be helpful, to inform yourself about what the other person is experiencing. For example suppose you were talking to a blind person; you get the basic idea of being blind but you don't know what it's like, so it's quite reasonable to wonder, say, how the person chooses groceries or decides what clothes to put on in the morning. When depression is chronic one develops a variety of coping strategies (which work more or less well at different times), and articulating that sort of thing often helps me get out of a trough and recover my sense of agency.

One other thing that's very common for people with depression is muscle pain, because the inner tension is often mirrored by a physical tension. It's hard to describe, but the physical tension can even act as a focus for the bad feeling. A backrub or a neck rub can go a long way, if the depressed person is feeling up to it. Other times one may not want to be touched and is better off just going to bed or somesuch.

Thanks so much, this is very helpful. I'll remember that asking questions is usually okay; I'm often afraid that patients prefer not to think or talk about it, or that I'm the 30th person asking about it and they're tired of it, etc.

The problem of depression is, simplifying, an inability to experience happiness. Telling a depressed person to cheer up is like telling someone with a broken leg to stop having a broken leg, or telling someone with lung cancer to just stop having lung cancer. Not only is it not helpful, it's insulting because it reveals a huge misunderstanding of the issue.

Entirely right. To be clear, I wasn't asking why "cheer up" is wrong, but what is right to say.