When supporting a friend with mental illness, it can be really tough to know what you should or shouldn't say and do. It's very much a learning curve, and I must stress that everyone is different! I'm going to write this particular post by focusing on what I need from friends who decide that they're going to support me.
Non-judgement - My illness can cause me to say or do silly things. This can cause backlash when people don't realise that what I've said or done is due to the fact that my judgement is impaired by my illness. In these cases, what I need is a friend who isn't going to tell me I was silly, but someone who reminds me that the whole thing will blow over. I need someone who'll stand by me until it has blown over.
Listen - For me, there's nothing worse than feeling like I'm not being heard. If your friend is saying something to you, listen as though it's the most important thing you've ever heard. You don't necessarily need to respond, a lot of the time just listening is enough. Nobody has all the answers, and whilst that can be frustrating for us, we're aware of that fact. We don't expect you to magically solve all of our problems.
Ask - Mental illness is a personal experience for each individual. We rarely meet textbook definitions, and as for internet resources...basically, we don't fit in boxes. Please don't Google our illness in an attempt to find answers to any questions you have. Just ask us and we'll be able to give you an answer that is specific to us.
Understanding - It can be very easy to lash out at people when you're walking around feeling like you're being consumed by darkness. Try not to hold that against us. We don't want to hurt you, we just genuinely cannot help it.
Company - My anxiety means I often avoid social situations. However, I still have the same basic need for human interaction as anyone else. Sometimes I just need to sit down with a friend and watch a film in silence. Sometimes I need to talk about random stuff. Sometimes I need to go for a walk with someone. Sometimes I just need some company.
I hope this helps if you find yourself supporting a friend with mental illness!