When specifying the position to walk to, you can use a node as the Y value of the position. This will cause the walkTo action to set the height of the avatar to the height of the terrain. Here is an example:
Code:
import viz
viz.go()
#Add terrain model
terrain = viz.add('tut_ground.wrl',euler=(0,-10,0))
#Create walk action that follows terrain height
walkAction = vizact.walkTo([0,terrain,10])
#Add action to avatar
avatar = viz.add('vcc_female.cfg')
avatar.addAction(walkAction)