In the "Advertisement" at the beginning of the book, Siddons defends his free adaptation of a work by Johann Jakob Engel. While a straight-forward translation of Engel's work might have proved "sufficient" Siddons argues that "as the application of his principles, in the original work, was adapted to the business of the German Stage, and as his references and examples were chiefly taken from the German drama, it became an essential duty of the Editor to Anglicise the matter, as well as translate the language of his author."
Siddons makes sure to give examples from English plays that were well known at the beginning of the 19th century, including Joanna Baillie's De Monfort, Matthew G. Lewis's The Castle Spectre, and William Congreve's Love for Love, as well as popular works by Shakespeare, such as Macbeth, Romeo and Juliet, and Hamlet. He also makes explicit reference to his mother. At one point, he writes: "If the first actress now on our stage had never been present at the bed of a dying person, her acting, under such circumstances, might probably have lost one of its most natural and affecting traits." This was Siddons's trick of having her fingers twitch, but nothing else, as she had once seen in a person near death.
How much was Siddons's guide used as a practical tool for actors? It's difficult to tell, but in addition to being raised by the most famous actress of the era (not to mention being nephew to the actor John Philip Kemble), Henry Siddons was a famous performer himself. He was the leading man at the Theatre Royal in Edinburgh, where he originated the role of the Lord of Lorne in Baillie's The Family Legend.
Actors today might not want to follow the advice in the book, which as its title suggests is aimed at a rather rhetorical approach to performance, but it is a fascinating glimpse into the past.