I'd say the vast majority of the people i've approached have been utterly fantastic.
There have of course been a few 'bumps in the road' along the way, won't be naming names for obvious reasons :-), but...i've had at least 1 person who wanted to use interview to promote himself/his company and completely unwilling to discuss what happened to any of the many abandoned games over the years.
A good few i myself have cancelled-in these cases, contact was made, questions sent over (and they do take a while to think up) then either nothing heard at all or i'd chase, be promised something by...nothing arrives and then you just move on.
If your lucky you take a sideways approach, try someone else who worked on project, get an alt. viewpoint.
The 1 area you do have to be careful of how you approach, is the lost/canned games..many people lost money, invested blood, sweat and tears etc into games that were canned through no fault of their own and it's a painful time to look back on.They are'nt the good old days, people were often working long hours for little pay, got screwed over for payments etc etc.It's still a very cut throat buisness out there.
I've just always gone in with an open and honest approach, explained who i am (a nobody) and who i'm asking questions on behalf of and why...and here's a few examples of past interviews and if you'd like to chat/can spare the time, we'd be delighted to hear from you......
And the responses speak for themselves i hope.
I honestly am eternally grateful to everyone from within the industry who went out of their way to answer my emails, provide source material like old concept sketches, photo's etc to go with the answers, to vouch for me to other contacts in the industry etc etc.
The fact others appreciate the interviews just rounds everything off nicely.
Sad side is so much material has now been lost due to storage mediums being thrown out, disk failures etc and many i'd loved to of put questions too have sadly passed away :-(.