In the UX toolbox there are variety of implements. Some aid a project, some delay it. It's up to the UX designer to determine which to use and when to pull in additional stakeholders to aid in the facilitation of brainstorming, workshops, and usability labs.
While working through my reading list, I hit page 43 of Mental Models by Indi Young. If you don't know what mental models are, I highly suggest picking up a copy of her book. Page 43 shows a great chart for determining when to use different user research techniques and starts by categorizing them into three high level buckets: Preference, Evaluative and Generative. Take a look below: