I want to comfort my sick friend without looking too emotional and sounds pity..

Take your cue from them, if they indicate they want to talk about their illness listen to them. If they do not want to talk sometimes just being their or giving them a hug or a good old hand holding is what is needed. It is not always necessary to talk, talking may be to painful for you as well as them. Look at the situation they are in, what do they need? Do they need help in any area? Food for example or running errands or helping with children or house. Each situation is different just always be thoughtful and kind. Try not to be pushy respect their need for privacy.

I have the same problem last year when one of my friend was got leukemia. I just visited him, talked about anything and pretended nothing happen. I don't know if I did the right thing, but that is life. I don't want to make him sad...

whats wrong with them?