(written from a Production point of view)
Brian John Williams is a stuntman and stunt actor who was the main stunt double for Brent Spiner on Star Trek: The Next Generation, four feature films, and Star Trek: Enterprise. He was also a regular stunt performer on Star Trek: Deep Space Nine, where he doubled actors such as Marc Alaimo, Rene Auberjonois, and Robert O'Reilly. He received no on-screen credit for almost all of his Star Trek television work.
One of his stunt scenes as double for Spiner's character Data in Star Trek: Insurrection was cut and removed from the final released version. This scene, a fight scene with stuntmen Brian Avery, Joey Box, and Eddie Braun, was explained and shown in the Star Trek: Insurrection (Special Edition) DVD feature "Anatomy of a Stunt".
Several costumes worn by Williams were sold off on the It's A Wrap! sale and auction on eBay, including a pair of boots worn in the episode "Extinction"  and the helmet from Star Trek: Insurrection.