I noticed on some shows on IMDB actors are listed, but as uncredited, especially for major actors.
Is this to do with the Screen Actors Guild, or for other reasons?
Peter Capaldi went uncredited for his first Doctor Who appearance (during the Matt Smith era, the Eleventh Doctor), but this was presumably to maintain suspense or something like that - I can't remember what.
It was interesting to note that Amy Acker appeared at least twice, uncredited, on the BBC's Holby City, once in 2015, once in 2016. However, she didn't get a listing in the end credits of Holby City. Since this show is a British production I'm not sure if SAG counts or not for this here. (I am from the United Kingdom, and watch a fair few BBC shows anyway)
Would this count as a cameo appearance, or as an extra?
I have no idea why the appearance was, but would be interested to know why some go uncredited.