I recently updated my jME3 and jMP from alpha-1 to alpha-2 and in the switch, my PhysicsCharacterNodes started to slide whenever I add a vector to their walkDirection. Is this common or do I need to apply friction somehow?
Uh. Its meant to be that way. Thats the purpose of the characterNode, to move in the walkDirection 
To have the character not move, set the walkDirection to zero.
Cheers,
Normen
It’s not the fact that it’s moving, but it’s sliding after it’s moved. I press down the w key to move forward, release it, and then my node goes sliding forwards until I hit either “a” or “d” (which turn left/right) then it stops.
Yes. As long as a walkdirection is set the character will move. Set it to zero to stop the character. The physics is framerate independent and runs at 60fps internally by default. Maybe you tried making it move faster forwards than backwards by increasing the forward vector length and so when the backwards vector is subtracted to stop the character a difference is left?
Cheers,
Normen