Referal interview handling tips

You did a mistake I think. You might have joined the IT job and then you can resign any time from that job if you got the core job. Unnecessarily you have lost a chance. Always remember that a bird in the hand worth two in the bush.

If you say that you have not joined because of selection for a core job which has not materialized, they may think that you are interested in an IT job and hence they may not give you another chance. So I think you should not tell the fact.

You can tell them that you have some health problems. You can say because of that problem you couldn't join the job and at the same time, you can tell them you have not written to them for an extension. They may believe and may consider you for the post again.

Give them a proper reason which they believe. One is health problems to you or to someone in the family or an accident in the family are the reasons you can tell them for not accepting the previous offer.

Considering the tough time in respect of getting a suitable assignment in the present market, it could have been a prudent step on your part to accept the IT job being proposed by the employer. However, you got offers from both IT company and from your own stream. Because of your passion in your own area, you did not respond to IT job. Now no response is forthcoming from your core area.
Take it as an easy way, sometime such a paradoxical situation may crop up causing confusion in the decision making process. You have now got an interview from the same IT company through the referral drive. The Board of interview may ask you a question why you did not accept to turn up for interview in the last time. You have to clarify the circumstances leading to your failure to turn up that time. Giving them the clear picture regarding your preferrance for the core sector would not be a favourable situation and so you need to be a diplomatic person giving some other reason to convince the board.
The reason may be the health ground of some member of your family and because of your involvement in connection with the treatment, you failed to turn up for the job in the IT company offered by them in the last time.
Your convincing reply and your better performance in the interview can lead to success.

If you perform well in the interview and you have good academic record there is no reason why they will reject you. If they ask you why you had not joined them earlier you can explain that you had some opportunities in hand and you wanted to assess and check them all before joining a particular job to avoid the hassles of resigning and joining in the other place. So after seeing all the opportunities you found that this particular job was of your choice and interest and you could do good contribution here and make a career. That is a simple statement rather than telling some excuse. Please remember that interview board members know everything about the candidates just by the way they answer their questions. So, I think a frank, honest and straight answer will be more fruitful than making some general excuse which many people will be telling there and the board members would not be oblivious of those general answers.

Attending an interview is more of a matter of confident mannerism with good knowledge of the subject. So, that is what matters there. Long back I attended one interview where I was asked how to measure current flowing in high voltage lines without breaking the lines. I told very confidently that there are tong testers used for it and it is so easy to do it. The next questions was whether at any time I had done that. My immediate reply was that I had not. I had only read that. So there was no further question about that and they switched to other questions. Please remember that you should not give them an opportunity of cross questioning you when they have a doubt on your statement. Talk correct things in a straight manner and avoid the cross questioning which is the worst part in any interview.

You had 2 opportunities in your hand out of which 1st was in the IT field and the other in your cour area and it is much understood that any engineer would like to work in his/her mainstream as it helps them to explore, learn and even put into practice what they have learned. Nowadays, we see many engineering pass-outs working in IT, Banking or other field but that is due to a good salary package or lack of opening in their branch.

As you were waiting for your core branch to provide you a chance of service but they never returned and the other chance is to hold on to the IT field due to the current scenario, you need to prepare well and expect the question of late reply to the application or selection. There are two options with you as other members have commented:
a) You tell them the truth but this may either go well with the interviewer or may back-fire on you or your selection. Many interviewers like the frankness of the candidate but as you are a fresher, I would not recommend you this as this may show your changing attitude and they may not like to take the risk of training you and if you get a better prospect, you may resign is what they may feel.
b) You need to find a solid reason with some supporting answers to the query. The interviewer may not be convinced at the first instance but you need to support your answer with some evidence or answer which can be justifiable and trustworthy. Never may they feel that you opted for the job on the secondary ground.

There are various reasons that can be attached like higher education, health issues ( it can be you or anyone in the family), Not in town, etc. Be sure that whatever reason you present, you need to be ready with some other questions related to it, so be prepared.

Now, prepare yourself for the interview with the IT company requirement like programing language, software or hardware, etc. Why opt for IT when you are from Mechanical branch, etc. Be confident, focused, well dressed i.e. presentable(Your first impression is the best impression), relax and listen to what they are asking, then reply in a soft, calm yet persuadable manner.

Though you have done some mess-up without understanding the priority, you can still try to manage to give successful interview by reasoning the 'referral' point itself.

You may also cover one of the above reason (given by other members) in case if it drags in interview. The main theme in the interview will be, confidence, self-introduction, appraisal if any, written test, mock test (if applicable), concern head interview, HR interview in final, finalization.

So, nothing to worry on this and give the casual answer with cool temperament. If not this job, you can get another job attitude should be there (effort too).