The actor told SFX that Coleman's new companion character - widely believed to be named Clara - will be "very different" to her predecessor Amy Pond (Karen Gillan).
"I think Steven's writing her in a very interesting way - in a different way to Amy Pond," he said. "I can't really tell you too much about her and her story because it's all top-secret as always... but we're very excited and we're planning to make the show with her bigger and better than ever and just keep moving forward."
Smith continued: "Jenna's doing a wonderful job, she's a wonderful actress and very dedicated."
The 29-year-old added that he will miss "great friends" Gillan and Arthur Darvill (Rory Williams), who will depart Doctor Who midway through the seventh series.
"I'm going to miss them terribly," he admitted. "[But] it is a show that is always evolving and by its very nature regenerates itself so you've got to... move with the times."
Doctor Who will return to BBC One in August.
