One of my best friends has anxiety and so do I.

She's one of the few friends in my life who has fully understood my nuances and triggers without me having to give a dissertation of my methods and thought process. We are both. If you are not as lucky to have this dynamic but you're someone with anxiety or are a friend to someone with anxiety, there are a few methods we have developed or came about organically:

We give each other space:

When we've had enough of the outside world we give each other space. We don't hold anything personal. If she's tired of facing the world, we check in, and then we leave each other be. When we've had sufficient recovery from the world we fall back in line like we talked every day.

We talk to each other as though we are counselors:

We listen, reiterate, and offer advice once we've established that's what the other person wants. If we just need want to vent we say "I just need to vent." or we ask the other "do you want my advice?"

We don't judge each other:

Of course, everyone has judgments about what they hear or see but we don't judge the other person's actions. If I did something that I shouldn't have done she allows me the space to be wrong and we talk through the issue. Then I can objectively see my error and how to correct. If we're being irrational we give space until we can see the issue for what it is.

We listen for key words or watch for actions that proceed anxiety attacks:

Our triggers are very different. There are certain phrases I use or a loss of articulation that shows when I'm death-spiraling. There are certain words she uses that let me know she has reached her limit.  We circumvent and start to intervene then we politely ask each other to stop, find something to focus on, and breath. If that doesn't work, because sometimes it doesn't, we ask the other to just continue to breathe. We speak in low tones and calmly until the attack ends.

We confirm one another:

We actively confirm concerns that the other has brought up. Either of us might say "correct me if I'm wrong" or "am I tripping" when presenting a concern.  The other unapologetically tells the other person the truth with respect and deference to the situation. When we've accomplished something we confirm each other's feelings and remove doubt of what is to come next.

We check before we "go in":

Because we both have anxiety we check with each other before we share information about our day, a situation, or how someone has just treated us. We ask through text "do you have a moment?" That's our cue that we need to be at full attention or if we need to say "not right now." Generally, we say "give me a minute" if we can't talk and we prepare to talk to one another later.

We respect each other's personhood:

Yes, we both have anxiety, but we are still different people with different lives. We generalize only to things we have defined as similarities. We don't underestimate the realities and responsibilities of each other's lives because we share similar experiences living with anxiety.  We share what we are experiencing, acknowledge each other's concerns, and respect our differences.

We love one another:

The friendship has turned into a sisterhood. We rely on one another to understand when we can't find anyone else to. We don't dump on one another but when our other friends and family just don't understand we have refuge in one another.

These are probably methods that are transferable to any relationship. But they are intentional methods we use to support one another.  These methods have seen us through very rough and traumatic events. They've also helped us developed intuitive skills that have benefitted our other relationships, namely our marriages.