When it comes to trying to be super-careful to say exactly the right thing (i.e. Mar-Bear/bikini situation) - I will absolutely fuck it up. I get flustered, inadvertently make an inappropriate joke, you name it. When I get anxious, it gets ugly, and I will usually say the wrong thing.
But when my brain switches into crisis mode - i.e. friend in distress, taking control of a chaotic situation, someone needs to be cared for - everything is calm. No stress, no anxiety, I just do exactly what needs to be done. Be totally supportive, listen, give feedback - even if it's something that directly involves me. The switch just flips, it's crazy.
It's weird, man, I dunno why it happens that way - but I feel like Marten's got something similar going on, like you said.
It's a strange phenomenon for sure. I'm about the same as well (except my life is more or less lived online, which does help when you can revise your thoughts before submitting them). Under normal circumstances, I'm pretty much either a stammering idiot, or silent, awkwardness and all. But when something is up with someone, you just... say the things you think they need to hear, and it's helpful.
In my case, it's probably because of that Internet "comfort zone" that I have. Get me face-to-face, and it's not there (my mouth hits "submit" before my brain finishes writing its message, if you will, and me thinking on the fly... does not work).
Also on today's comic, I d'awwwed a little (OK, fine, a lot) at their interaction today. Bro still has to look out for his sister, trans* or not. Siblings: The only people that will torment each other for their own amusement, and then beat the shit out of anyone else that tries. At least, ideally so...