Doctor Livesey is one of the primary characters in Treasure Island by Robert Louis Stevenson, and he is one of the "good guys" in this novel. Squire Trelawney has devised a plan to go and seek the treasure with the map Jim Hawkins has, and he invites the doctor to go with him.
"Trelawney," said the doctor, "I'll go with you; and I'll go bail for it, so will Jim, and be a credit to the undertaking. There's only one man I'm afraid of."
"And who's that?" cried the squire. "Name the dog, sir!"
"You," replied the doctor; "for you cannot hold your tongue.
While the two men trust one another on a personal level, they must keep silent about the specifics of their plan in order to avoid trouble. It is Trelawney's job to hire the crew for this voyage, and Doctor Livesey is afraid that the squire will "blab" about the reason for their upcoming trip.