Agyeman, 27, was called "an exciting new talent," by "Dr. Who" writer Russell T. Davies, Hellomagazine.com reported.
Agyeman played seductress Lola Wise in "Crossroads" and has also made appearances in "Casualty" and "The Bill."
"The search for a new companion had been under way for some time when I first saw Freema," Davies said. "Watching her filming confirmed what an exciting new talent she was."
She sealed the deal during her audition with "Dr. Who" star David Tennant, Davies said.
"They are terrific together," Davies said.
As for Agyeman, she told the Web site she never in her "wildest dreams" thought she would wind up on "Dr. Who."
"It still hasn't quite sunk in," she said.