Post-Interview Thank you Notes: Internal interview?

Everyone knows that post-interview thank you notes are important, and hand-written notes are better than e-mail (at least in my non-tech field).

However, my question is related to internal interviews.

1. Do I have to mail thank you notes through the post office to people in the same office as me? Or is it appropriate to send it interoffice? Or personally hand to the interviewer?

2. What if I'm interviewing for a position in a group with whom I already have a good relationship with (i.e. go to happy hours, meet daily at water cooler/coffee machine, work four offices down from each other and interact with professionally often)? They're not close friends but definitely people that know more than a little about each other's personal lives. It seems somewhat strange to write a formal note thanking them and rehashing my skills, accomplishments and good fit for the job. Would a more informal note be better, and if so, how should I phrase it?