Double Star by Robert A. Heinlein
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 Smythe's own life was on the line - for if he wasn't assassinated, there was always the possibility that he might be trapped in his new role forever!