It is a bit hard to see, you mean the neck and head bending in ways you did not expect? I fiddle with the location of the head and neck tags myself. I tend to put the neck tag at the base of the neck (on a handle) and the head tag up closer to the bottom of the head. Adjusting their relative position changes the body posture up there. I don't have an exact reason why, I just fiddle and play and see if I can get it to look better. He sometimes stoops more than expected when they are not "right". Also the Strength properties of the walk behavior can help sometimes too.
I don't use the walk behavior very often though, so never spent the time working out good debugging practices. But its usually relative position of handles that makes a difference. Shoulders, waist, hips, etc. I try fiddling until it looks better. (Oh, the yellow mesh view (icon at bottom of scene window) sometimes helps as well - if it shows a strange distortion at a spot that gives a clue which handle to try moving next.)
Oh sorry for not being specific enough. As seen in the first photo, the top half of the body extends too forward (technically, too much right) while the head is too further back (too much left). Thanks for the tips! I'll try them!