PA-28 pilot reported runway excursion during landing rollout.

Narrative

I was the pilot flying (sole manipulator of the controls) in the left seat. In the right seat was the acting PIC (as I was not current for passengers). I kept a slightly higher airspeed on short final due to wind conditions. Upon landing in winds that; at the surface; were reported as 12KTG17 with a 10-degree right crosswind; I found the aircraft to be difficult to control. It swerved a bit left and right. As I attempted to gain control I found it nearly impossible to add left rudder when needed. Then; suddenly; the left rudder freed itself and the plane swerved severely left; entering the grass momentarily before I turned back onto the runway. No damage was done. The acting PIC asked me 'What do you suppose happened there?' I answered that I believed we were both on the rudder pedals. He replied that he did; in fact; have right rudder applied. I suspect when he released right rudder pressure; the left rudder became free and; since I was applying significant pressure on it; this caused the incursion. I had debriefed with the PIC before the flight; indicating if he said 'my controls' that I would relinquish them immediately. However; he had not indicated to me that he was planning to use the rudder during our landing; nor did he inform me he was on the rudder. I have learned that if I have an acting PIC with me and I am the pilot flying; I must be more specific about communication and use of the controls.