David Tennant is most known among Doctor Who fans as the Tenth Doctor, who first appeared onscreen in 2005, but the actor who brought Ten to life had already been involved with the franchise years earlier. He played a character in an earlier Doctor Who story from 2003, when the show itself wasn't even on the air.
Tennant has returned as the Doctor twice since his 2010 exit as the show's leading man. His first comeback saw him reprise his role as Ten in 2013's "The Day of the Doctor," and he also surprised everyone by taking on the role of the Fourteenth Doctor in 2022's "The Power of the Doctor" and the trilogy of episodes that followed. However, it wasn't just Doctor Who's 50th and 60th anniversaries the Scottish actor helped celebrate — he was there for the 40th too.
David Tennant has an uncredited Doctor Who role in 2003's Scream of the Shalka
Originally released weekly on BBC's website to mark Doctor Who's 40th anniversary, Scream of the Shalka was an animated miniseries that featured Richard E. Grant as the story's main character. While initially intended to be the Ninth Doctor, Grant's version of the Time Lord is now referred to as the Shalka Doctor, and his place within the franchise canon is nebulous at best. Regardless, Tennant has a small role in the project that predates his casting as the Doctor and the 2005 revival of the show itself.
Tennant's voice can be heard as the Warehouseman in episode 5, Scream of the Shalka's penultimate installment. He also had other small roles in Doctor Who audio plays that same year. His earliest Doctor Who role came in 2001 as Feldwebel Kurtz in Doctor Who: The Monthly Adventures, marking his first official connection to the franchise. However, it wouldn't be until Christopher Eccleston's departure in 2005 that Tennant would step into the role of the Doctor.
Another pre-Tenth Doctor role set up David Tennant to lead Doctor Who
Doctor Who was arguably Tennant's big break as an actor, despite working in the industry for years before he was cast as Ten. That said, a project that aired the same year as his live-action franchise debut also had a huge impact on his immediate professional future. Starring as the title role in 2005's Casanova miniseries, Tennant found himself collaborating for the first time with Russell T Davies.
The three-episode run allowed Tennant and Davies to form a winning formula between writer and actor, and it's something that has served them both well through the years. Almost immediately after Casanova, Tennant stepped in to replace Eccelston when the latter surprised Davies by announcing his desire to leave. If Tennant hadn't already proven how successfully he could work with Davies, their Doctor Who partnership may never have happened.