Double Star

Double Star by Robert A. Heinlein. mass_market edition. ISBN: 9780345330130.

One minute, down and out actor Lorenzo Smythe was -- as usual -- in a bar, drinking away his troubles as he watched his career go down the tubes. Then a space pilot bought him a drink, and the next thing Smythe knew, he was shanghaied to Mars. Suddenly he found himself agreeing to the most difficult role of his career: impersonating an important politician who had been kidnapped. Peace with the Martians was at stake -- failure to pull off the act could result in interplanetary war. And Smythes own life was on the line -- for if he wasnt assassinated, there was always the possibility that he might be trapped in his new role forever!