Let’s take a step back and look even more closely at the basic activities that are happening within the conversation.
One activity that's obvious is that the players, Sandra and Mark, have adopted the roles of their characters and take action as if they are those characters in the fiction.
While the example doesn’t show the GM adopting any roles, it is the case that he too will do so regularly, whenever Lonnie and Billy encounter other people in the fiction. For example, if Joe Exotic is really inside the trailer, the GM will adopt Joe’s role, while Mark and Sandra's characters interact with him. We call the characters whose roles the players adopt player characters (PCs), and characters whose roles the GM adopts non-player characters (NPCs).
Associative & Dissociative Mechanics
The second thing that's happening is not so obvious, and it has to do with how the PCs' action relates to the mechanics the GM selects to resolve their chance of meaningful failure: the dice roll Sandra makes is associated with Lonnie's chances of safely sneaking across the enclosure. Justin Alexander of The Alexandrian calls this an associated mechanic:
In order for a game to be a roleplaying game (and not just a game where you happen to play a role), the mechanics of the game have to be about making and resolving choices as if you were the character. If the mechanics of the game require you to make choices which aren’t associated to the choices made by the character, then the mechanics of the game aren’t about roleplaying and it’s not a roleplaying game.Justin Alexander, “Roleplaying Games vs. Storytelling Games”
He goes on to argue that roleplaying, then, is a combination of players adopting the roles of characters whose action is resolved by associated mechanics. This is why a "game of pretend" is not an RPG—there are no associated mechanics, so while it is roleplaying, it's not a roleplaying game. Theater acting is also not a roleplaying game, for the same reason.
To illustrate what "association" means for mechanics in RPGs, Alexander references the game Dread, in which players adopt roles whose actions in the fiction are modeled by Jenga blocks: when a character tries to do something in the fiction, the resolution mechanic is the act of the player drawing a Jenga block from the tower. If the tower collapses, the player is out of the game.
Per Alexander's distinction here, Dread would not be a roleplaying game, because while players adopt roles and take actions as characters in the fiction, the mechanic that resolves their action is dissociative: the mechanic of a player "pulling a Jenga block" is in no way related to what the character is actually doing in the fiction.
Justin's example of an associated mechanic is even more striking: when a character casts a fireball spell in various editions of Dungeons & Dragons, the amount of damage the fireball deals to its target is equal to a number of dice reflecting the character's adventuring experience. The damage mechanic is associative because it reflects action the character is taking and attempts to model that action in a way that reflects the reality of the fiction.
If you read about Dread on the web, you'll find that the game is generally described as an RPG. Are people simply mistaken as to what Dread is? Many a Dread player would argue that the game is unquestionably an RPG, and would bristle at the idea of it being set aside as a "story game." So then, is the presence or absence of associative mechanics enough to distinguish RPGs from other games? Or are associative mechanics a single quality of RPGs, among others, that together make them distinct from other games?