Yes, those are to be considered fragments, or possibly elliptical sentences, because, as you say, they lack a main verb and aren't complete sentences. The cast is listed, so this is best treated as a list; in lists, sentence fragments are common, especially noun groups, as here.
The capital at the beginning of the fragment and the full stop at the end aren't uncommon either outside of full sentences; they seem normal to me in this context.
This list and the way it is phrased are somewhat informal, yes. Semicola can be used between fragments rather than full clauses, especially in an enumeration where the enumerated phrases are long or contain commas, in order to make the sentence easier to read. That is not the case here, but somehow the semicolon does not surprise me. I would say that a semicolon can also be used between fragments if the whole text is made up of fragments rather than full clauses, which is the case here. Sometimes, this will be a sign of informality. But compare the title of an academic paper, which is hardly informal, but which may include a semicolon (a title is often also a fragment):
Double Titles
For an alternative or double title in English beginning with or, we
follow the first example given in section 8.165 of The Chicago Manual
of Style and punctuate as follows:
"England’s Monitor; or, The History of the Separation (452)"
— https://style.mla.org/punctuation-with-titles/
