Ugh. Lots of altruism [DRAT, "idealism" is the ism I meant} isms goin' on in here. Well, let me say how it's happened to me twice.
Back when I was a CFI building time hopping on passenger requested "two pilot" flights in order to build multi time while I worked the radios (ok back then — I think), I went on a trip to DTW on an insanely low IFR day. This was before CRM, cell phones and Sully. We broke out at minimums for 21R and had to taxi to Butler from the furthest place on the airport. Of course, I wasn't familiar with the airport, but the PIC was, so he said. So, when we touched down, the passengers began applauding. As we taxied, they started to ask how we could find the runway in all this fog. Well, even I could see the PIC ought to be paying attention to the taxi route rather than talk to passengers, so I took it upon myself to try to satisfy their curiosity. Then, I heard the ground controller screaming our N-number to stop immediately. I turned my head back forward just in time to see a DC-9 lower its nose gear and deploy the TRs as it flashed before us in the intersection. We stopped just past the hold short line. The PIC had a lot of splainin' to do, writing letters for about six months. Me? I learned a lot and passed it on to everybody I ever flew with from that day forward.
It doesn't just happen to GA 135 flights, though. Many years later, at MDW, I was a B-727 SO jump-seating to see my honey. I can't remember why I never forgot, if it was the close call or the mean captain who ran his seat all the way back to cramp my toes and discourage me from ever thinking about jump seating a 737 ever again. In my life. But as soon as he firewalled the engines the FO told him to abort the takeoff as the tower canceled takeoff clearance for a landing air carrier on one of the cross runways. That one could've been really bad if in all the rising cockpit noise level the message didn't get through.