I think I've identified a possible reason why the rider/horse pairs came into the new scene slightly out of position. It has to do with the dreaded "keyframe drift."

I always leave everything in its default location/pose on Frame 1 and do all my posing on Frames 2 and up (depending on how many shots I need per scene). After I set up the Parks & Trails set in a new scene file, I loaded one rider/horse pair from the figures library, which came into Frame 1 already posed. Then I loaded the other pair via Import/Poser Document (b/c I hadn't saved them to the library), and they came into Frame 1 in default poses, with the riding pose on Frame 2.

I used Frame 2 to translate the rider/horse pairs to the right spot on the trail, and then I think I must've advanced to Frame 3 to fine-tune poses. What I forgot to do was to copy Frame 2's keyframes to a point further down the timeline (like Frame 5) and to set the keyframes to Constant. So the reason their hands were no longer properly gripping the reins on Frame 3 was most likely that their bodies were starting to drift along a Spline path back to their default poses. (And I started fixing that before I thought to check hand positions one earlier frames.)

I'm pretty far off of my original question by now, but since it's just come up: is there a way to make Constant the default type of interpolation for all new files? I've gotten myself into more than one crazy posing mess due to the way Spline interpolation works!

PS to 3D-Mobster: I never use IK for anything and always have it off, both b/c I prefer to pose w/o it and b/c it messes up how poses get saved. I did try using it to keep the riders' hands connected to the horses' reins, but it didn't work as intended. Neither did using "add constraint" or parenting, probably b/c the reins are part of the bridle figure rather than being a separate prop. (And I'd lose all the helpful morph dials if I converted them to a prop.)

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TOOLS: MacBook Pro; Poser Pro 11; Cheetah3D; Photoshop CC

FIGURES: S-16 (improved V4 by Karina), M4, K4, Mavka, Toons, and Nursoda's people

GOALS: Stylized and non-photorealistic renders in various fantasy styles