Double Star

Double Star is a science fiction novel by Robert A. Heinlein first published in 1956 as a serial. It's The Prisoner of Zenda In Space

Lorenzo Smythe is an out of work actor. Brilliant at his trade, egostistical as hell, but completely unemployed. He receives an interesting job offer: impersonate Bonforte, a politician, to hide his kidnapping from the public. To pull the deception off, Smythe must learn to completely become another person in looks, thoughts and actions and put aside his own.

Although Smythe is reluctant at first, he soon becomes interested in the political opinions he's now forced to advocate, and starts to make an honest effort to get along with the aliens and the political supporters he always tried to avoid in the past. And when he has to make an appearance at the court of Bonforte's long-time friend, Galactic Emperor Willem of the Netherlands, he finds himself enjoying his new role and the friendships it brings more than he'd imagined.

Tropes used in Double Star include:

Author Filibuster: It wouldn't be a Heinlein novel without one. Particularly, Smythe's long internal monologue to himself in mid-book as he tries to understand Bonforte's political philosophy and ethics. Justified in that its his job to impersonate Bonforte, and he's trying to better understand the 'character' he is portraying. Especially since he now has to impersonate Bonforte to the Senate, which means being able to convincingly give the man's political speeches.
Blue and Orange Morality: Martians have a highly complex system of politeness. The main problem of the book is that a politician may be late to a ceremony that inducts him into a Martian clan. There is a legend on Mars about a young Martian who was late to something important, and the consequence of this is death. He was given a second chance, on account of being young and having only a partially formed brain. He would have none of it, so he brought a case against himself in court, successfully prosecuted himself for being late, was consequently executed, and is now held in reverence as the patron saint of propriety on Mars.
Bluff the Impostor: The Emperor figures out Smythe is not Bonforte when he agrees to play with his toy trains. Bonforte and the Emperor had a friendly in joke between them: the Emperor would always invite him to play trains and Bonforte would always refuse and make fun of his hobby.
Lost in Character: Smythe is hired to impersonate a kidnapped politician. He becomes so immersed in being this man that after the original is killed, he takes over and actually becomes him. By the end of the book, he's happily abandoned his old life. What's more, his secretary has convinced herself that she never loved anyone but Smythe, and Smythe doesn't really mind the fact that she's obviously lying to herself.
Method Acting: Smythe used this to get into his characters, and even used the psychotic aspect of one character as a crutch when he had to chop up a body.
Noodle Incident: Smythe has many of these in his background:

"He mentioned a couple of details in my past that I would have sworn were buried and forgotten. All right, so I did have a couple of routines useful for stag shows that are not for the family trade — a man has to eat. But that matter about Bebe; that was hardly fair, for I certainly had not known she was under age. As for that hotel bill, while it is true that bilking an 'innkeeper' in Miami Beach carries much the same punishment as armed robbery elsewhere, it is a very provincial attitude — I would have paid it if I had had the money. As for that unfortunate incident in Seattle — well, what I am trying to say is that Dak did know an amazing amount about my background but he had the wrong slant on most of it."

The Power of Acting: Smythe's acting skills are all that stands between Human and Martian relations completely falling apart.
Secret Identity Identity: When Bonforte dies, Smythe must choose between his own identity, and the greater good.
Small Name, Big Ego: Smythe, especially early on in the novel. He's entirely convinced that he's one of the finest names in theater, and his lack of recognition is mostly a case of rotten luck. At the end of the novel, when he looks back at how he was before the events of the novel took place, he can't believe the bloated sense of self worth he used to have.
- Zig-zagged in that Smythe is shown throughout the novel to be an unbelievably talented actor, in addition to being a highly talented make-up artist. Prior references are also that he's a skilled stuntman, juggler, and sleight-of-hand expert. Its honestly puzzling as to how he could fail to have a successful career. Either his agent was horribly inept, he was that talentless at selling himself, or his luck bordered on Diabolus Ex Machina.
Snowball Lie: The basis of the entire novel.
Spotting the Thread: Smythe spends weeks studying Bonforte's files on everyone around him to make the masquerade work. He notices that the file on the emperor is nearly empty, and assumes that they don't have much contact... until he meets the emperor, and gets found out, because the point of the files is to help Bonforte remember things about people less politically important than he is. He's caught when he dutifully plays with the emperor's toy trains instead of teasing him about them.