The core purpose of the prototype, the assumptions you are trying to validate/invalidate or the knowledge gaps you are trying to fill should have all been discussed in Converge. During this phase you will build a quick and dirty prototype. Since you have limited time to build the prototype it should be as low-fi as you can get away with during Testing.
During this phase roles shift. Designers are typically the ones building the prototype unless it is low-fi enough for everyone to contribute. Product Owners should be in charge of getting realistic information, data and copy into the prototype. It should be clear what everyone’s role is before the phase starts. The team should check in with each other as much as possible to make sure everything is on the right track.