(written from a Production point of view)
Christopher Doohan (born 28 June 1959; age 54) is one of the sons of Montgomery Scott actor James Doohan. Chris and his twin brother, Montgomery, made uncredited appearances as background extras in the first Star Trek feature, Star Trek: The Motion Picture. They can both be seen among the Enterprise crewmembers gathered for Kirk's briefing of the V'Ger threat.
Chris Doohan is primarily a singer. In this profession, he competed in a televised talent show known as Your Big Break in 1999, in which players must impress a studio audience by impersonating the musical celebrity whom they most look or sound like. Doohan emulated country singer Garth Brooks on the show, which was produced and aired by NBC. Doohan also works as a Registered Vascular technologist.
The 2009 DVD box release Star Trek: Original Motion Picture Collection featured a reunion of fan extras in the "Special Star Trek Reunion", including Doohan, David Gerrold, Bjo Trimble, Jo Ann Nolan, and Fred Bronson chatting on the same soundstage used for the rec deck scene.  During this chat, Doohan mentioned his connection to J.J. Abrams' Star Trek and that the fitting reminded him of the days in 1979.
Upon hearing that his late father's role of Montgomery Scott would be recast for J.J. Abrams' Star Trek, Chris Doohan has started a campaign asking fans to write to Paramount Pictures in an attempt to persuade the studio and the filmmakers to allow either him or his brother, Montgomery, to play their father's role in the film. At the very least, however, they would like to be extras in the film as they were for The Motion Picture. 
The role of Scotty in the new film went to British actor Simon Pegg, but on 5 November 2007 it was announced that Christopher Doohan had auditioned for a "minor speaking role" in the film. Doohan stated he would not have even won an audition had it not been for the support of the fans.  In January 2008, it was announced that Doohan had indeed won a role in the new film.  He appeared alongside Scotty, assisting him in operating the transporter controls during the mission to the Narada (stating on Twitter that his character was Kyle).
With the announcement of the Star Trek Into Darkness, fans once again began a campaign to bring him back to the Enterprise. It was announced on IMDB that Chris will play a transport officer in the film. It was also announced Trekmovie.com that Chris did get a speaking role in the new film, and that Simon Pegg was instumental in making that happen. 
In 2012, it was announced that Chris Doohan will play Scotty in the upcoming fan film series Star Trek Continues, a fan film series that has been met with serious controversy since it was announced on trekmovie.com.