Yes, there's something about lying down that makes you feel more vulnerable - of course I'm thinking lying face up with the goop coming down on you from above. Maybe it's because you can't as easily move away. Also the goop has no where to go but on and around you, so you tend to get more messy for the same amount of goop.
I recall seeing a GIF of somebody tied down spread eagle and having buckets of mud poured on their face. All they could do was move their head back and forth a bit, without much relief. That was definitely one of my "wish it were me" moments.
We did a couple of scenes where the girls received their entire messing while lying down, in this one Chastity and Kitty, both in nurses uniforms, take turns to mess each other up lying on the table: https://gungemastercom.umd.net/download_info/medical-mayhem
Our dungeon table died this summer, but we're looking for a replacement, possibly one suitable for chaining people down, to really submit to their slimings.
Love you, too
