Was it a mistake to act more interested in a potential advisor's work than I actually am I am a prospective PhD student who is applying to several schools.

A couple months ago I spoke with an advisor from a school I was somewhat interested in.  We talked for quite a while, and he seemed really interested in taking me on as a student, to the point where I'm pretty confident about it.  

The thing is, especially after researching his background, I'm really only interested in his older work; the project he's working on now doesn't excite me nearly as much as I acted like it did.  I the excitement of speaking to a professor from a prestigious university made me so enthusiastic.  

I like much of this Professor's other work, but he seems really passionate about this project and I'm unfortunately not.  I'd be willing to work on it a little, but I couldn't imagine dedicating myself to it.

I guess I really have two questions:


If I did get into this University, how likely would I be to have to work on this project?  Either by getting a different advisor or by working with this professor on something else.  Obviously this depends on the situation, but maybe there's some general advice that applies?
How do I go forward?  I don't want to pretend to be interested in something I'm not, but I definitely don't want to ruin my chances.


Thanks for any advice.
There is something missing here: If you talked to him for a while. I suppose part of this talk should address your research/interests and what you are going to do as a PhD student. Does that mean you acted as if you were interested just for the sake of acceptance? Was it an involving discussion or a one-sided one (the professor talks most of the time)? 

If you spoke about your interests and he seemed interested. Then most likely he has something for you within your interests. Even if he is working on other projects.


  If I did get into this University, how likely would I be to have to
  work on this project? Either by getting a different advisor or by
  working with this professor on something else. Obviously this depends
  on the situation, but maybe there's some general advice that applies?


You always have the option of opt-out of the subject and changing the advisor. This happens regularly specially for first year PhD students. 


  How do I go forward? I don't want to pretend to be interested in
  something I'm not, but I definitely don't want to ruin my chances.


This depends upon what stage are you in. If you still did not apply to the school, try to be honest and write your research statement exactly as you wish. Try to line the professor research with your interests. IF you did apply, then there is nothing you should do except waiting for the decision.