How many times has an actor/actress done a guest spot *on screen* and then later joined the series - as a new character? Here's the ones I could think of ...
Lala Ward - was Astra, then became Romana 2.
Colin Baker - was Maxil, then became the Sixth Doctor.
Freema - was a Torchwood agent, then became that character's cousin, Martha.
Karen Gillan - was one of the soothsayers, then became Amy Pond.
Special nomination for Nick Courtney who was Bret Vyon, then became the Brig.
Jacqueline Hill would not count for this. She was a regular first (Barbara) before coming in as a guest, Lexa.
Are there any I've missed?
Lala Ward - was Astra, then became Romana 2.
Colin Baker - was Maxil, then became the Sixth Doctor.
Freema - was a Torchwood agent, then became that character's cousin, Martha.
Karen Gillan - was one of the soothsayers, then became Amy Pond.
Special nomination for Nick Courtney who was Bret Vyon, then became the Brig.
Jacqueline Hill would not count for this. She was a regular first (Barbara) before coming in as a guest, Lexa.
Are there any I've missed?