Would this be a good idea...???

if you can do it in your head why would you bother?

Good point, but the model is a bit more detailed, and allows you to run different scenarios. It is like walking through a DCF analysis vs. showing a DCF model. But in the big picture, the point is to show that you really want the job. And doing that little extra step.

no, makes you sound crazy insecure/trying way too hard

Seriously? If you got it right, there is no reason to follow up with further analysis. It'll just make you look like a try hard.

People just want to hear the right answer, not how you got there. My boss doesn't care how I structure my models or build things out, he just wants to know the assumptions and what they shit out. They trust you are doing things properly, otherwise you don't last long.

The OP's question is quite a bad one, but let's make it more interesting.

What if you answered a question incorrectly in the interview but then could show a model and give a good answer? This would show your dedication as well as your ability to quickly master concepts that you may not immediately understand.

Doesn't matter. no one has time to look at some piss poor excel file made by some college kid trying to get an internship. they already decided on who got the job by the time you get to the airport.

Definitely wouldn't do that. Also, I have disagree with tiger2012's statement above - I think people care more about your thought process than your actual answer. No matter what level you are in banking, we spend a lot of time defending our assumptions and ideas. The output of your model obviously has to be within reason, but there's a lot of gray area in finance.

Ok, that settles it.