I've probably seen people crying on bed in every position. But when we're talking, we're both usually on our sides or on our back. I'd always paid more attention to the tears more than body movements, but I don't think I've seen an adult cry hard enough for the bed to actually shake.
The most memorable one was this girl I was interested in was laying on my bed, opening up to me about something. We were on our sides facing each other, and I noticed tears pool on the corner of her eye, and then slipped off the bridge of her nose and dripped on my pillow. Then another tear followed and dropped on the same spot. She wiped her eyes and turned on her back and I noticed 2 tear stains on the satin sheet. Then another drop of tear came down the side of her eye and I took the opportunity to wipe it with the back of my finger. It was a nice time