Inspired by this question on SciFi&Fantasy.SE. The background is that Terry Farrell, an actress on the series Star Trek: Deep Space Nine, left the show on bad terms after the sixth season. The following year, the series finale featured several montages of clips from old episodes, but Farrell's character was notably absent, despite her long involvement with the show.

When asked about the reasons for this, pretty much everyone agrees that it was a matter of permission; they needed Farrell's permission to use the clips, and couldn't get it. In the words of Ronald D. Moore, one of the show's main writers:

In order to use a clip of someone from a previous episode, you first have to obtain permission from the actor in question.

I'm inclined to take Moore at his word, since he is involved in the industry and I am not, but I was surprised to read this; I would have thought the studio would retain full rights to use their old episodes in any way they see fit.

So, I was hoping Law.SE could help clarify my understanding. What legal grounds (if any) are behind this requirement?

^{Aside: since I have only a passing familiarity with the legal profession, plain English explanations of any citations is greatly appreciated}

Inspired by this question on SciFi&Fantasy.SE. The background is that Terry Farrell, an actress on the series Star Trek: Deep Space Nine, left the show on bad terms after the sixth season. The following year, the series finale featured several montages of clips from old episodes, but Farrell's character was notably absent, despite her long involvement with the show.

When asked about the reasons for this, pretty much everyone agrees that it was a matter of permission; they needed Farrell's permission to use the clips, and couldn't get it. In the words of Ronald D. Moore, one of the show's main writers:

In order to use a clip of someone from a previous episode, you first have to obtain permission from the actor in question.

I'm inclined to take Moore at his word, since he is involved in the industry and I am not, but I was surprised to read this; I would have thought the studio would retain full rights to use their old episodes in any way they see fit.

So, I was hoping Law.SE could help clarify my understanding. What legal grounds (if any) are behind this requirement?

^{Aside: since I have only a passing familiarity with the legal profession, plain English explanations of any citations is greatly appreciated}