Clarifying the seemingly obvious: Cast and credits

Cast and credits appear to be simply relationships between films and people, and perhaps fictitious characters. Is it really that simple?

From: The Tales of Hoffmann, UK, 1951.

This is Frederick (later: Sir Frederick) Ashton acting as a character named Cochenille, taken from a German manifestation of the British production of Hoffmanns Erzählungen, made in 1951. The first thing to note is that his contribution is, like that of most actors, at the work level.