For me, the direct interference comes up most when I'm directly dealing with a stressful situation. "I'll just bring my journal along and work [in the hospital cafeteria|in the car on the way to the hospice|in the hotel before the funeral]": no. I've said that to myself at least once on all of those occasions, and sometimes I got a few words, a mini-scenelet out, but mostly, fiction writing is not something I do well with under that kind of direct and immediate stress. (On the other hand, my journal has a sort of security blanket effect, so I feel better even if I get no fiction written.) Also, if I'm sick enough that I can't/shouldn't get out of bed, I don't get much fiction written. Ideas generated, yes, but stories written, not so much.
If I'm worried about a lot of stuff but nothing is active at the moment -- I'm still waiting for news or waiting for a doctor appointment or something -- well...that's kind of how life goes. You slop the hogs, you write the stories, you do things while you wait or nothing ever gets done, because there's always something going on somewhere. There's always something to stress about.
(Sometimes I think the sense that there's always something going on stops me from looking at what I actually have going on. Every once in awhile I do have to step back and try to look at these things objectively, or I will fall over and wonder why I fell, because, gee, it's not like there's anything going on.)
I think I'm lucky in that I don't have the sense that I write better or worse under stress. I have no incentives to create stress and drama to feed the stories, because the stories do just fine without it. I also have no incentive to huddle in a corner and swear that I won't be able to do anything worthwhile anyway, so I might as well not try. I know some people navigate those pitfalls just fine, but I'm glad they're not mine.
Also, two things: I am done with the fortnight of menstruation! Oh hurrah! And you know how I was short on underwear? I bought some new ones and promptly stuck my thumb through one pair the very first time I wore it. Luckily, I hadn't bought any others like it, and won't again. It was not a "Hulk smash!" moment. It was just...poorly constructed underwear, apparently. We do not approve.