Before they were famous
...Or well known, or somewhat successful or something...
Most of the main cast were nobodies, except Pat Roach, who'd appeared in Kubrick's Barry Lyndon as a hardman wrestler and had minor fame doing the same thing in real life. And Gary Holton who had minor Rock 'n' Roll fame. Later most of them went on to become fairly successful, some more so than others.
Series 1
* Ray Winston as Colin the Deserter. I think he appeared in Scum before this.
* Michael Elphick as Magowan. A few years later he would appear in Boon, and become a household name in UK for that.
* Jenny Eclair as a surprisingly good looking German waitress in the hotel. (And whose German is not bad either.)
* Sheila Reid, Mrs Bomber, appeared in 35 episodes of Benidorm apparently.
Maybe an honorable mention for Lysette Anthony (Christa), but I think she was already well known at this stage. She later posed for Playboy.
Brigitte Kahn (Dagmar) and Michael Sheard (Herr Grunwald) had fairly good careers before this. Sheard played Nazis a lot, and someone in Grange Hill!
Series 2
* Melanie Hill as Hazel Redfern, seems to have gone on to a pretty full career after this.
* Denise Welch as Jean, the woman in Oz's former flat. Later Mrs Healy, and ex-Mrs Healy. Went on to bigger things.
* John Bowler as Howard Radcliff, the architect. Turns out he is in 229 episodes of the Bill!
* Sammi Davis as Pippa, the polytechnic girl Barry scares off.
* Sammy Johnson (Martin Cooper, the thug Moxey fights with) went on to appear in twenty episodes of Spender.
Honorable mention for James Booth (Kenny Ames), who had been in a lot of films before this, but had a recurring role in Twin Peaks for several episodes.
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It's not "sci-fi", it's SF!