Jeffrey Jones did not receive an Oscar nomination, despite being nominated for a Golden Globe, for portraying Emperor Joseph II.
Anyway with that out of the way onto Jeffrey Jones as the Emperor of Vienna Joseph II considered the musical King who wishes Mozart (Tom Hulce) to become part of his court, even though many of the other musical officials of Vienna dislike Mozart including the jealous Salieri (F. Murray Abraham) the official court composer for the Emperor. Amadeus is a film really with many enjoyable smaller roles filled well by various character actors, but Jones perhaps does the most with his part as the Emperor. His role is not a big one but important to the film.
Jones plays the Emperor as not a man of great power, or necessarily even a man of great substance. Jones finds a very particular way of portraying the Emperor as a bit of a doofus, well at least when it comes with the official musical dealings of the court. Jones is quite good though because he puts a great deal of wit into his portrayal that really could have been far more of an over the top idiot, but instead Jones finds just the right tone in which to play the part.The most important part of this is by taking this approach to the part he frankly is far funnier.
It is an entertaining performance particularly in his short little moments of showing the Emperor's ineptitude like when he struggles to think of a word to describe how he felt about Mozart's first opera for him, and merely takes to heart the suggestion of one of his underlings. Importantly though Jones does not overplay the stupidity of the Emperor, as there are other times where he believably shows himself to at least be a leader of some sort. In the way he carries himself, and addresses others he does suggest the right undercurrent of power in the man, but as well as a certain welcoming quality within in.
Jones finds a way to play the Emperor as he should be by supplying both laughs in displaying the Emperor's lack of musical knowledge, but as well that this man is an Emperor after all. He is particularly very good in the scene where Mozart pleads his case to be able to put on the Barber of Seville. He does well in conveying a genuine interest in the artists endeavor, but as well just the right power in his oppositions. As the Emperor states his own case Jones is convincing in that he is not opposed because he is stupid, but rather it does stem from a genuine concern as the leader of his people. This is not an especially complex part, but Jones handles it just right and adds quite a bit of joy to his film.