Costars Billie Piper and Matt Smith would both go on to feature in Doctor Who (2005) - Billie Piper played companion Rose Tyler (2005-2006; 2008; 2013) and Matt Smith played the Eleventh Doctor (2010-2013).
The trivia item below may give away important plot points.
Both Billie Piper and Matt Smith starred in the TV series Doctor Who (2005-) but not at the same time. Billie Piper played Rose Tyler, the companion to the 9th (Christopher Eccleston) and 10th (David Tennant) Doctors in the first two seasons. Matt Smith played the titular role as the 11th Doctor for seasons 5-7. The two starred together in the 7th Season 50th Anniversary Episode "The Day of the Doctor".