How can watching be non-doing?
MN1 The Root of All Things addresses watching for the untrained ordinary person as:


  They perceive the seen as the seen. But then they identify with the seen … Why is that? Because they haven’t completely understood it, I say.


In contrast, for the trained mind SN35.96 we have:


  ...in the seen will be merely the seen;...


The simplicity of "merely the seen" is itself non-doing. Ordinarily we identify with thoughts, latch onto them with greed, aversion or delusion. That latching onto thoughts is the doing to be relinquished. Yet we perceive the latching onto thoughts as "not-doing", hence the seeming paradox. 

As we practice, we start noticing the doing, the grasping. And with a measure of watchful restraint we counteract the tendency towards "doing". From AN3.60:


  When there is sense restraint, one who has sense restraint has fulfilled a vital condition for ethical conduct.
  When there is ethical conduct, one who has fulfilled ethical conduct has fulfilled a vital condition for right immersion.


Find the razor's edge between doing and not-doing. Explore that.