The play- the idea

The play would be about free will, and what neuroscience has shown so far on it. It would be about thoughts running multiple random simulations of reality to make a simple decision, and then consciousness lying about why the decision was made. Each of the thoughts would be a different scene, seemingly unconnected, but the underlying theme would be about the decision. Some will be very short, interrupted. The "real" world would develop in scenes, in between.
The people who really decide are the ones living in those thoughts, the random simulated reality scenes. The protagonist in the real world simply rationalizes his decision, believing it was his/hers to make. However, he/she does have the free will of saying "no" to the decisions made by his/her thoughts.

The decision could be about any decision, but it won't be interesting if it's too small of a decision, unless the "simulation" scenes are more exciting. E.g. a life and death decision, or a complex romance decision in the simulated world, leading to a simple decision of buying flowers to say I am sorry, is one type of thing. Another, would be, the reverse, kinda. Making a major decision in end, with the option of saying no. Suicide would work here, but literally any major decision, its rationalization and the power to say no, would also work. Endless possibilities.