Doctor Who has been a sci-fi staple for the past 60 years. Originally starting in the '60s, it made its revival in 2005, ready to attract viewers old and new. One perk of The Doctor's regenerative abilities is it helped keep the show going with a new actor being able to slip into the Time Lord's shoes whenever it was time for the previous actor to leave.
Source：https://www.cbr.com/doctor-who-forgotten-actor-cameos/
