Doctor Who was criticised in the 80s for casting American and Australian companions, as some critics felt that it was too obvious an assault on overseas markets. Rose Byrne however is an Australian actress capable of a brilliant English accent, as demonstrated in I Give It A Year. At 34, she's older than most companions, which could provide an interesting dynamic in the TARDIS, especially if she's not all that impressed with a man, just because he has a time machine. She's also fantastic at comedy, she has great timing, and she's an excellent character actress; it's a shame she's probably staying in Hollywood.
23. Gemma Chan
Having already had a small role in 'The Waters of Mars', Gemma Chan is slowly becoming more and more well-known. The girlfriend of Jack Whitehall, Chan has played shy, vulnerable characters (Dates), and bitchy, brash ones (Fresh Meat); she's also used to playing characters younger than her age (31), and is waiting for a huge role in television. Everything points to becoming a companion one day, and she's definitely got the likeability to succeed as one. She's another actress who's also appeared in Sherlock, as well as Secret Diary of a Call Girl, alongside a certain Billie Piper.
