Most of the interactive designers that I've met originally came from the visual arts: illustrators, animators, print design. It figures. In the screen-centered environment we tend to think of "user interface" design as a graphical skill that delivers a checklist of good design attributes.
Mandated to record functional specifications, tech writers are usually most interested in being helpful: creating accessible, understandable end user guidelines, training, online help, etc. Because of our organizational and communication skills, we are often the de facto usability advocates on the development team.
In the technical development environment, artists and documentation writers traditionally carry little weight. However, the Business Unit champions the customer relationship, knows the customer best and is a strong lobbyist for "usability".
Cognitive studies probably acccount for most of the credentialed "usability lab" folks. Although originally few in number, they can be exceptionally influential, bringing academic rigor, testing methodologies and professional credibility to the mix, along with a focus on the dynamics of interaction.
Product testers are the "tecchie" version of the behavioral scientist. QA is generally more concerned with functional conformance to the workflow than customer satisfaction, but - because of our process-orientation - we are also vigorous champions for "best practices" & standards.
True enough to form, my background includes strong graphics (I had been a commerical artist and animator) and documentation / training (a degree in educational media). I've done a lot of marketing presentations for new media ventures and roll-outs. I honed my writing, editing & librarying skills as a Documentation Manager and Technical Writer.