Did Roald Dahl write and appear in every episode?
No. Although the series was originally envisioned as a show televising the stories from his "Tales of the Unexpected" and "More Tales of the Unexpected" short story books, the popularity of the show led to stories from other authors being used. The episodes based on other authors stories had no introduction at all and by the third series, the title of the show no longer included Dahl's name.Neither did Dahl write the screenplays for the show. Dahl's stories were adapted by Ronald Harwood, Robin Chapman and Kevin Goldstein-Jackson.
Was there an American version of the show? I recall seeing American episodes.
No. There are episodes filmed in the US using American acting talent. These episodes are generally easy to spot by the opening and closing credits being of a darker image of poor quality. It has been suggested that this was down to the conversion process involved between PAL and NTSC formats. This is unconfirmed.
Haven't I seen these stories on other shows?
Quite possibly. The original "Alfred Hitchcock Presents" show used some of Dahl's stories. Also, some stories used on that show were used on "Tales of the Unexpected". These include:The Landlady,
The Man from the South,
Lamb to the Slaughter,
Mrs Bixby and the Colonel's Coat,
Never Speak Ill of the Dead ("De Mortuis" in Alfred Hitchcock Presents)