Dreaming about celebrities in general can mean you're subconsciously thinking about personal accomplishment. This could be because you've recently been doing really well and are congratulating yourself mentally, or because you're really trying to get to that point.
However, the meaning can be more specific depending on the people you're dreaming about. It sounds like this is the case for you as they're celebrities playing characters, so you feel like you know more about them. In this way, it can be read more as a dream about people you may know.
One theory is that in dreams, people represent the part of themselves that you see in yourself. So if you were dreaming about Monica in Friends, you may be associating with the determined or picky part of yourself.
Dreaming about the different parts of yourself reflected in these celebrity characters could mean that you're happy with those bits of yourself.
However, acting in dreams has been said to reflect a part of you that's pretending. So if you were dreaming about Monica from Friends, her most obvious attribute being orderliness, it may be that you're making yourself be like that, maybe to impress other people.
Dreaming about being in a relationship with a celebrity could represent the way in which you'd like an intimate situation to play out in real life. As if your unconscious mind is running through the situation like a television show to see how it could go, with the different types of characters.
It's really interesting to think about dreams as they can often help you work out how you truly feel about things you've been unsure of.
However it's important to remember that it could just mean you've been watching a lot of celebrities on TV and your mind is showing you things you've been thinking a lot about. Especially if you've been imagining yourself in a relationship with the characters while awake.