Johnny, the issue of payment to participants is a big concern to all of the studios and as a rule they simply will not do it because if they pay one person they are setting a precedent for having to pay everybody. A little bit of history here:
In the time of the Old Republic, there really was no such things as special bonus features or behind-the-scenes materials. There was only the MOVIE that mattered, and everything else was PUBLICITY/MARKETING. If there were royalties to be paid, they were paid on the sales of the movie alone, so any money spent on marketing or PR was an expense that went against the profits of the film. The more you spent on PR, the less profit the movie made, and the longer it took for profit participants like actors to make any money (and we are going with the fantasy that there are such things as net profits in the film industry, which of course cases like Buchwald v. Paramount disproved). Consequently, the studios had made a deal with the Screen Actors Guild and other orgs that anything to do with publicity or marketing should be done by actors for free, since it arguably got the actors closer to receiving residuals sooner. In other words, any actor who isn't helping to promote the film is sabotaging the film and him/herself. Thus, SAG does not demand that actors be paid for footage seen in trailers and featurettes (below a certain running time), in interviews or other publicity-related things. At the time, no one made money off special features or marketing... only the success of the box office mattered. You weren't able to sell PR materials (except to fans at conventions, of course, and this was considered illegal by most in the biz).
Then came laserdisc and Criterion and the idea that marketing campaigns were a component in the history of a film. Trailers and other PR materials --some of them more creative than the films themselves-- began to be included on discs. Items that were meant solely to promote a film were now being considered valuable additions to a home video release and might in fact get more people to buy the video. At the beginning of this, SAG maintained their contractual position that their actors needed no additional compensation for PR work... provided that the work was actually used for promoting sales of the film and was not in fact a saleable item in itself. This meant that if you put a trailer for the T1 film on the video of T2, it's MARKETING because you are trying to get people to go and buy the T1 video; but if you put the T1 trailer on the T1 video, it's now a SPECIAL FEATURE that's being sold directly, since you have to buy it to see it, and it's not promoting the T1 video any more. So SAG rules basically meant that if you wanted to include the T1 trailer on the T1 video or laserdisc, you had to pay every actor seen in it for the "non-PR" use of their likeness... even if it was from the final film! This is why the original laserdisc of Aliens had no Aliens trailers on it, but the Abyss laserdisc could have Aliens trailers on it.
The only way to get around this issue was to track down every actor seen in a trailer and get them to sign a clearance letter stating that they agreed not to get paid for the "non-PR" inclusion of the trailer on the video release of the same film. As a rule of thumb, most actors agreed to this under what is called "favored nations" status... that is, each actor would agree to waive any fees as long as ALL other actors on it did the same. Now, as things beyond trailers started showing up on discs, like behind-the-scenes materials, CREW MEMBERS started showing up and they had to sign clearance letters as well. If they chose not to do so, we had to edit them out of pieces or blur them so they couldn't sue what they considered "deep-pocketed" studios for "unauthorized" use of their likenesses.
With the institutionalization of special features on DVD as the norm, these issues have largely been dealt with from the outset by the studios now routinely having a clause in the contracts for ALL cast and crew members that basically states that the contractor or employee acknowledges that there will be documentation of various sorts of the processes in creating the film and that they agree that any footage of themselves that might be captured in the process of their work will be the sole property of the studio and that they agree that the studio can do whatever they want with such material. Actors fall into this now as well, with some exceptions for high-end actors who have right of refusal for material that shows them in a bad light (and even if they don't have that legal right, a studio wouldn't burn that bridge with a star anyway).
So when it comes to doing interviews or commentaries, it is generally understood that no payments will be made for participants to do them. If someone demands money to do it, then you won't be seeing them on the disc. This will often explain the curious lack of certain parties on discs out there... you can decide for yourself. Now, the question is, do you feel that their lack of participation makes the studio look bad for not paying them, or does it make the non-participant look bad for holding out for money when everyone else participated for free?
A number of years ago, Lionsgate (back when it was Artisan) wanted to get Arnold Schwarzenegger to do a commentary on Total Recall with director Paul Verhoeven. Arnold wouldn't do it unless Artisan paid a large sum of money to one of his charities (this is one way to get around the payment issue). Artisan paid something like $75K to get Arnold's participation, and the entire industry was freaked. Many studios called it the death of special features, because if the studios had to pay for participation on new features, then there would be no more new special features... the cost would be considered too great for the, since --let's be honest-- practically NO ONE buys a movie on disc for the special features alone. The movie is the key item, as it should be. What basically happened is that, thank goodness, the Total Recall situation was seen as an anomaly, not a trend, and it did NOT start a wave of actors and crew refusing to participate unless there was payment. Most people understood that it was better to be seen on the disc and associated with the film than to be absent and whispered as being only about the money. some guilds, like the directors and writers guilds, have tried to demand that their members be given the right and opportunity to record commentaries and participate in special features for no remuneration... they just want to be seen as the creative forces behind the film (in fact, the writers' demand came up only after the directors got their demand fulfilled).
So now, pretty much no one gets paid to participate, and only really big stars or directors have the clout to force a studio through noncontractual means to deviate from that (mostly by withholding happiness in working with that studio again). Most participants just want to know that no one else is getting paid either, and they are happy with the free publicity they get for an hour or two of their time. On older films, some potential participants may feel they've been cheated of profits or other opportunities over the years and as such will choose not to participate in something even though it may seem to be in their best interests to do so; in this case, it's more about trying to punish the studio in some manner. Some actors have even tried to bargain by refusing repeatedly and hoping the studio will cave and pay them, but this almost never happens. And in a lot of cases, it's the agents, managers and spouses who have been the gatekeeper of access to an actor who is perfectly willing to participate. But studios don't budge on this issue much, because not only are they constantly weighing the costs versus profits to their shareholders, but they know that giving in on one thing means possibly having to open the floodgates and pay everyone. They also know that if anyone asks, they can put the blame on the nonparticipant for wanting money when everyone else participated for free, so it's not the studio who's being unfair (or else no one would have participated).
Hope that provides a long-winded answer to your question...
V