Imagine the following scenario:
- Four adventurers are traveling from one city to another. They have 1 horse which is pulling a covered wagon.
- One adventurer is riding and controlling the horse, two are walking alongside, and the fourth is resting in the wagon (not sleeping).
- The party is ambushed by a group of monsters. Let's assume that all the PC's are surprised and only the monsters take their turn. However the monsters do not know that there is a fourth character in the wagon.
- The two characters who were walking begin fighting the monsters. The character on the horse stops it, dismounts and grabs his weapons. He does not begin fighting because he has already used all of his actions interacting with objects.
- Meanwhile, the character in the wagon hears fighting and decides that he should join in. He gathers his weapons etc and gets ready to fight.
- A few rounds later, the character jumps out of the wagon and attacks one of the monsters.
What happens? Does the attacking player get advantage? Is there some sort of surprise round? (I know there are no surprise rounds in 5e, but you get the point) Does nothing happen because the monster is already in combat and alert to all threats, including unexpected ones?