Theory:

INTRUDER: I’m not taking it for fun. I’ve been hunted long enough. I’m wanted for murder already, and they can’t hang me twice.
 
GERRARD: You’re planning a gratuitous double, so to speak. Admitted you’ve nothing to lose, but what have you to gain?
 
INTRUDER: I’ve got freedom to gain. As for myself, I’m a poor hunted rat. As Vincent Charles Gerrard I’m free to go places and do nothing. I can eat well and sleep and without having to be ready to beat it at the sight of a cop.
 
GERRARD: In most melodramas the villain is foolish enough to delay his killing long enough to be frustrated. You are much luckier.
Explanation:
 
The intruder told Gerrard that he was being chased by the police as he had already committed a murder, so if he killed Gerrard, the punishment would remain the same and he would not be hanged to death twice for the same reason (murder).
 
The phrase here, "gratuitous double", implies the meaning that, the intruder had already committed a murder and was planning to commit another murder by killing Gerrard, which Gerrard thinks would be unnecessary. Gerrard advised the intruder that he was planning to kill him for no reason. According to Gerrard, though the intruder would not lose anything by killing him, he would not gain anything by murdering him. Gerrard felt so because he did not know the intention of the intruder behind killing him. And so, he conveyed this logical question to the intruder.
 
Gerrard was curious as to why the intruder wanted to kill him. And the response Gerrard received from the intruder was "freedom." It was because, according to the intruder, if he killed Gerrard, he would get freedom as he had already committed a murder and was hiding himself like a hunted rat. As Gerrard was an introvert and unfamiliar to his neighbours and tradespeople, the intruder thought to use that opportunity.
 
So, upon killing Gerrard, the intruder could use Vincent Charles Gerrard's identity and live a life with more freedom. He could go out to many places, eat and sleep without the fear of being caught by the police. Now we can understand why the play was named "If I Were You," as the intruder wanted to kill and use Gerrard's identity. Gerrard was shocked to hear such an explanation behind the murderer's motive.
 
Gerrard observed that in many dramatic plays, the villain is shown as a fool who would delay the murder of the victims. Eventually, as poetic justice would have it, his plan would become unsuccessful, with the victims escaping and him getting punished for his crimes. Gerrard then added that the intruder, on the other hand, has better luck than such villains.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Hunted To try to find somebody
2.
Gratuitous Unnecessary
3.
MelodramaA story or film in which the characters shows strong emotions than real people do
4.
FrustratedFeeling or expressing distress and annoyance resulting from an inability to change or achieve something
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.