They will say the minimum required by the story, like a "Yes" or a "No" to accept a quest.
Perhaps your player needs a little nudging into the right direction. You'll have to guide him there, and you'll see that this can be surprisingly simple. Allow me to elaborate.
When you are misunderstood, you aren't clear enough
When I studied journalism, I had a professor tell the class that "when somebody misunderstands you, it is because you weren't clear enough." Now, that may not always be right, but it's a good philosophy to go by: when somebody misunderstands you, you have to reword things so that they will understand what you meant.
Stretching this a bit, we can conclude that most of the time, when you get a wrong answer, it is because you asked the wrong (type of) question.
Types of question
Basically there are two types of questions: open questions and closed questions.
Closed questions are questions that typically only allow for yes/no answers. It's easy to recognize them: in English, they usually start with a verb.
- "Do you accept the quest to retrieve the Sword of Infinity +1?"
- "Is there any beer left in the fridge?"
- "Have you seen the new Quinton Tarantino movie?"
- "Does your father work at the library?"
- "Did you go to college?"
All of these can be answered with a simple yes or no, or perhaps such non-answers as "I don't know."
Open questions are the more interesting type of questions. A simple yes or no won't do for an answer, and some thought will have to be put into a response. You can recognize most open questions because they start with "who", "what", "where", "when", "why", or "how".
- "How would you go about finding the Sword of Infinity +1?"
- "Why would the Sword of Infinity +1 be useful for you?"
- "What did you think of the new Quinton Tarantino movie?"
- "Where does your father work?"
- "What was your major in college?"
The answers to these questions will tell you a lot more. The first two examples will let your player think about the quest beyond such trivialities as "do I accept it?"
And even in those cases where the answer is rather simple ("my father works at the library"), you can just throw more open questions at them ("oh yeah? What does he do there?" -"Well, he sets up programs to help children who have difficulties learning to read.")
In many cases, even if the answer to an open question doesn't seem satisfying, you can use one simple word to go a little deeper: "why?"
Alice: "What did you think of the Quinton Tarantino movie?"
Bob: "I liked it tons!"
Bob: "Well, as usual, the violence was over the top but in a different way than what we saw in his previous work, the dialogues were amazingly brilliant, blablabla..."
If you find that your player has difficulties getting that feeling of immersion -even if just a little bit-, you can help him think about the game, the world, his character, by asking open questions.
Don't ask, "do you want to go find a treasure in the Cave of the Dragon?" Instead, ask, "what would you like to do with the vast amounts of wealth that'd be yours if you conquer the Cave of the Dragon?"
If the answer is still kind of dull, go deeper by asking why:
They: "I'd buy a bigger sword, I guess?"
They: "So I can beat baddies better!" (+1 xp for alliteration)
You: "Why do you think your current sword can't handle it?"
They: "Last time I barely got past those redshirts. I just want to decapitate them!"
Ah. We've established that your player has found his bloodthirsty side. Good! Let the RPing commence!