Theory:

INTRUDER: Do you run a car?
 
GERRARD: No.
 
INTRUDER: That’s a lie. You’re not dealing with a fool. I’m as smart as you and smarter, and I know you run a car. Better be careful, wise guy!
 
GERRARD: Are you American, or is that merely a clever imitation?
 
INTRUDER: Listen, this gun’s no toy. I can hurt you without killing you, and still get my answers.
 
GERRARD: Of course, if you put it like that, I’ll be glad to assist you. I do possess a car, and it’s in the garage round the corner.
 
INTRUDER: That’s better. Do people often come out here?
  
GERRARD: Very rarely. Surprisingly few people take the trouble to visit me. There’s the baker and the greengrocer, of course; and then there’s the milkman — quite charming, but no one so interesting as yourself.
 
INTRUDER: I happen to know that you never see tradespeople.
Explanation:
 
The intruder then moved on to his next question. He asked Gerrard whether he had a car, to which Gerrard replied "no." However, the intruder refused to accept the response since it appeared that he already knew Gerrard had a car, as we could see the intruder urging Gerrard not to lie. He also told Gerrard that the intruder was wiser than he appeared and so he cannot be played as a fool.
 
Gerrard's car.jpg
Gerrard's car
 
Gerrard did not respond to him. Was the intruder truly an American, or was he imitating an American accent? asked Gerrard instead. Hearing Gerrard ask such a question enraged the intruder. He cautioned Gerrard that the pistol he possessed was not a toy and that he might use it to injure Gerrard in order to obtain all of the information he needed.
 
Gerrard answered the intruder that if he had warned him like that, he would be terrified and inclined to provide him with the answers. The intruder then asked if many people came to his residence often, to which Gerrard responded that that was not the case because he only received visits on rare occasions. No one else cared to come to see him except the baker, greengrocer, and milkman.
 
The intruder wasn't convinced by Gerrard's answer. He revealed to Gerrard that he knew Gerrard would never meet tradespeople.
 
We were able to deduce one thing from the intruder's conversation. The intruder must have known certain facts about Gerrard and watched him, his actions, and his surroundings at some point in the past. And, after learning of Gerrard's details, the intruder could have resolved to murder him in order to conceal Gerrard's true identity. The intruder wanted to clarify certain things about Gerrard before killing him.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Assist  To help
2.
RarelyNot often
3.
GreengrocerA person who sells vegetables 
4.
CharmingVery pleasing or attractive
5.
TradespeoplePeople whose job involves selling goods
6.
Garage A building for keeping vehicles
Reference:
National Council of Educational Research and Training (2006). Beehive. If I Were You - Doughlas James (pp. 138-144). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.