I've never experienced the misfortune of having a drink get spilled onboard a HST. However on one particular journey onboard a VTEC HST from Edinburgh down to London Kings Cross, I had my phone out and placed on one of the seatback tables and at one point (somewhere between Dunbar and Newcastle) my phone got violently catapulted off the table and onto the floor. Luckily the phone was unscathed but it wasn't the best experience to have.
Compared to my previous Class 91 experiences on the ECML, I find the HSTs tend to jolt and sway quite a bit more than the Class 91s, although that's just from my own personal experience. I do prefer the interior of the VTEC (now LNER) HSTs over the Class 91s however, as they seem to be more spacious and airy, although that's just my opinion.