Portable camera recommendation for interviews
Often, the best option, particularly for interviews, is a DSLR.  The particulars of your needs may change that or harden in on a specific level of DSLR, however in general they key thing for interviews is to be able to have a sharply focused subject with a nice blur to the background.

DSLR cameras shooting HD video with a fast lens (f/2.8 or faster preferably) can achieve very nice background blurs and make great looking video.  Most also have a line input that you can run a mic into, though use of the best quality mics will often require using an external device first to deal with the mic and convert it to the type of signal the camera wants.

Personally, I shoot with a Canon 5D Mark iii and use a Zoom h4n for audio.  I record the audio separately and sync in post production, but a line out from the Zoom can also be fed in to the camera directly.  You don't need something as advanced as the 5D Mark iii, but it hopefully gives you an idea of how a setup can work.  You could substitute any DSLR with a mic or line input for the 5D Mark iii.

If portability is super key however, it might be best to go with an actual camcorder.  They can get a little bit smaller than a full fledged DSLR though the smaller the camera, the harder it is to get the background blur that you often see in interviews.