Doohan played "Scotty" in the '60s TV series and in several movies.
"I tried to approach the role like he did. He's a Scottish engineer who works in outerspace. He's a bit of a brawler. I didn't try to facilitate change," the 38-year-old British actor said.
In "Run Fatboy Run," which opens Friday, Pegg wears short-shorts as an fumbling Londoner who begins training for a marathon to impress his ex.
He's more coy about what his Montgomery "Scotty" Scott will look like in the J.J. Abrams-directed "Star Trek," expected on the big screen next year.
"I couldn't possibly tell you what I wear in `Star Trek,'" Pegg said.
Pegg said he finished shooting the film, which stars Chris Pine as Capt. James T. Kirk and Zachary Quinto as Spock, last week. Pegg said he's signed up to star in three potential "Trek" movies.
"I'm looking forward to doing that for the next few years," he said. "Hopefully, I'll do a lot in between, too."
Other celeb attendees at the "Run Fatboy Run" premiere included Pegg's co-stars Thandie Newton, India de Beaufort and Harish Patel; Oscar winner Diablo Cody; Jemaine Clement and Bret McKenzie of HBO's "Flight of the Conchords"; and John Krasinski, Eddie Izzard and Jonathan Silverman.
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