An iterative approach is one where the content of the discussion, stimulus, or sometimes even the methodology is adapted over the course of the research programme. Learning from initial research sessions is used to influence the inputs for subsequent interviews. For example, in a creative development project the ad agency will amend scripts and storyboards overnight based on early feedback.
This approach is particularly useful for time-sensitive projects where there isnt scope for multiple rounds of research.
This is an idea that has been adapted from the validated-learning approach of the lean start-up movement.