I've found very very few crews who aren't more than happy to talk shop with an interested spotter. I owe the B-Med crew one a few years back who, after I sent a note and a photocopy of my license up to the skipper, got me up for a jump seat take-off from LHR, answered all my rubbish questions and then got me back for a night landing at Beirut. Fab!
Sadly not possible these days, or ever again in all likelihood.