The actors are on strike, so that means that a majority of them are not working. Therefore, instead of Nathan Fillion walking the beat on The Rookie, he was walking the line
outside a studio.
And since he is not filming, he can grow whatever facial hair he wants, so he grew a mustache. Now, he looks more like a cop than he does on his ABC procedural.
Hopefully, AMPTP will come to the table and agree to give the writers and actors what they deserve ASAP. This way, everyone can get back to work, and Fillion can shave that thing above his lip.