 The actor Dennis Christopher, who plays "Jack", is never credited as Dennis  Christopher, the credits just say "and jack". This is because the identity  of Jack is a secret (for the characters in the series, they dont know who  he is), his identity is unmasked in the first two episodes of the 4th  season, from this moment he is credited as "dennis christopher as jack" in  the credits. (There are two episodes, where he is credited in his role as  "Albert Newquay", in fact, "Albert Newquay" is "Jack", also revealed in the  meantioned episodes).