Theory:

INTRUDER: I’m O.K. I’ve got a reason for everything. I’m going to be Vincent Charles Gerrard, see. I’ve got to know what he talks like. Now I know. That posh stuff comes easy. This is Mr V.C. Gerrard speaking. (Pantomime of phoning, in imitation cultured voice.) And that’s not all. (He stands up.) Get up a minute (Gerrard stands.) Now take a look at me.
 
GERRARD: You’re not particularly decorative.
 
INTRUDER: No! Well, that goes for you, too. I’ve only got to wear specs and I’ll be enough like you to get away with it.
 
GERRARD: What about your clothes? They’ll let you down if you’re not careful.
 
INTRUDER: That’ll be all right. Yours will fit me fine.
Explanation:
 
Gerrard hinted that the intruder's stupid idea of murdering and taking Gerrard's identity would get him into problems. He also pointed out that it would eventually land him in the hands of the cops, whom he had managed to avoid for several days.
 
The intruder responded that he wasn't bothered because he was doing it for a reason. He wished to live under the mask of Vincent Charles Gerrard, to rescue himself. He was also certain that the cops would not notice him since he could replicate his sophisticated body language.
 
The intruder imitated Gerrard's mannerisms, which he picked up through overhearing Gerrard on the phone and from observing Gerrard's body language. He was attempting to establish himself as a capable Gerrard replacement.
 
However, Gerrard said that his impersonation was not very appealing, implying that he did not behave or talk like Gerrard, to which the intruder responded that it was because of Gerrard's behaviour. The intruder then told Gerrard that his impersonation was comparable to Gerrard's because he believed that having the same physique as Gerrard and his observation on Gerrard's tone and voice modulation would assist him.
 
He went on to say that he just needed spectacles to look like Gerrard, to which Gerrard responded by asking what he would do about the clothing, pointing out that if he didn't dress like Gerrard, he would be in trouble. The intruder answered calmly that it wouldn't be an issue because he could use Gerrard's clothing.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Posh Fashionable
2.
Pantomime An act of expressing thoughts through movements rather than speech
3.
DecorativeAttractive
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.