in rehearsal
1. In the act of rehearsing for something, typically a performance of some kind (such as a play). The director is in rehearsal right now, but I can give him a message.
2. Being developed through rehearsals, as of a performance that is not yet ready to be performed for an audience. That play has been in rehearsal for months and months—will it ever be seen by an audience?
in rehearsal
a stage of development in the production of a play, opera, or concert, involving many rehearsals. The play is in rehearsal now and will open next month. While the opera was still in rehearsal, the star developed a hatred for the director.