Generally Interactive activities require human involvement to complete their task. Only after the user completes the Interactive task associated to the activity, the instance moves to the next flow object in the process flow.
The Engine processes Interactive activities using interactive threads. If the implementation of the Interactive activity is of type screenflow, then the engine releases the interactive thread at the beginning of the screenflow and obtains a new interactive thread when the user completes the screenflow. When possible you should use screenflows to implement Interactive activities because they optimize the use of interactive threads.