The phenomenon of "sketch covers" primarily coming from Marvel Comics these days present quite a few problems for anyone doing the "sketching"...
First there's the size issue. Everyone may not know this but the average modern age comic book art page is 11 inches by 17 inches and many grow accustom to drawing at a much larger size than your typical comic book. So you have to plan ahead so that your figures aren't too big for the page. Luckily, I go for the wraparound effect as most sketch covers have a blank sketch area on the back page.
Secondly, there's the logo and barcode and pricing trade dress to deal with. It's tricky, but not a show-stopper and can even be used in the sketch.
Thirdly, there's the "I better not screw this guy/gal's comic up" fear. It's not as bad for me, as I work in a comic shop and can at least reorder a book if such a problem arises, but even that's not a full-proof thing.
Then there's the usual stress when dealing with drawing something for someone and that's all performance, which, it turns out, is the least troublesome. Usually, whatever crap we "drawers" turn out tends to be appreciated if for nothing else, the effort.
The above are examples of the trials and tribulations of doing a sketch cover... it ain't all smoke and mirrors. To enlarge a view on any of the above images, simply click on the image in question.