Where can I change mixing of an action in NLA? I combined two action strips in the NLA-editor but the forward movement fades in (red curve in the image below)  I would prefer to have this linear. Where can I change this behaviour? Action blending and extrapolation doesn't seem to have an effect.
The relevant setting is Blend In/Out. To ignore keyframes in strips below the active one, turn off Auto Blend In/Out, and set both blend values to 0:



More generally I suggest, for bones that will be animated in a layer, to avoid keyframing it in strip actions in all other layers. Or do it in such a way that the strips don't overlap. That'd save you some headache when the animation mixing gets more complex, especially if there are semi-overlapping action strips. Not used carefully, action blending could cause jerky movement that's hard to tweak.