Krzysztof Siwczyk was considered for the role of Andrea when director Charles Dance saw him in a re-run of Wojaczek (1999). Sadly, he then attended a Q&A session and deemed him to have grown up too much since the film.
In the film, which is set in pre-World War II 1936, the sisters go to London to see Andrea play in concert and meet him afterward. In the novel, which is set in pre-World War I mid 1910s, after Andrea departs Cornwall for London the sisters never hear from him again.
While Andrea is playing at the local party, another violin appears in the lower right hand corner and is being played at the same time that Andrea seems to be playing, but there's only one violin heard at the time. Could it be that the "real" violinist was captured on film by accident?