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!