Let's look back at Series Two of the modern era of Doctor Who, Russell T. Davies' second season, and David Tennant's first season as The Tenth Doctor. It was here that the series' shift into a global phenomenon began, and Tennant became the face of the modern era of Doctor Who in the same way Tom Baker is still associated with the classic era of the series. Yes, hardcore fans would prefer to say William Hartnell was the true face of the show or Jon Pertwee was their Doctor, but casual punters still think of Baker, partly because he was on the show longer than anyone else.
Just as Davies cast Christopher Eccleston as the Ninth Doctor after working with him on the miniseries The Second Coming the year before, Tennant had starred in Davies' cheeky, comedic, postmodern version of Casanova in 2005, the year Ecclestone premiered in Davies' revival of Doctor Who. There Tennant carried a whole series for the first time, playing a sexy, roguish rake, and seemed a natural to take over when Ecclestone left after only one season.
Tennant already had Science Fiction credentials: he had spent years playing bit parts and supporting roles in Big Finish audio dramas, though it would be decades before he played the Tenth Doctor on the audios. He played the dimension-hopping secret agent lead in Big Finish's audio adaptation of Bryan Talbot's The Adventures of Luther Arkwright (and will return in the decades-later sequel). He played a doctor in BBC Three's 2005 live broadcast remake of Nigel Kneale's The Quatermass Experiment, and he was on set in between scenes when he got the call that he had been cast in as the Tenth Doctor.
Davies and Tennant leaned into portraying The Doctor as a sexy geek, which went down a treat with a new generation of teenage female fans discovering the show for the first time. It was his Doctor that truly made the series popular with female viewers, on top of how many of them identified with Billie Piper's Rose. To have Tennant and Piper together was a double whammy of Secret Sauce. For male fans, Tennant made being a geek cool – he was cocky, cheeky, a chick magnet, and the smartest smart aleck in the room. What's not to like about this Doctor? Davies could use Tennant's popularity to start making really big swings with the show and establish many of the modern show's tropes that are still being used, possibly overused in some instances. It was here that the new Doctor Who really began to become the show it was meant to be. No wonder Davies brought him back to relaunch the show on its 60th Anniversary this year before he passes the torch to another new era.