They're basically unrelated. The actor paradigm is basically imperative programming, with boundaries being put up between the actors, and the way you cross the boundary is with a message. If that sounds too foreign, bear in mind that while OO doesn't have to be an "Actor" and an actor doesn't have to be "OO", they share a certain structural similarity, in that one of the primary characteristics of working with an "object oriented" system is that you are creating imperative islands of code, and the way you cross the barriers are via method calls. Indeed back when OO was first being formulated you can see the terminology was not settled yet, and you'll see method calls being called "messages" even today in some OO communities.

The Actor model is basically OO, except that objects are given their own execution contexts, whereas in OO there's one execution context that drives the objects. You can do worse for your first design in Erlang than to break your task into core objects exactly as you would for a high-level OO design (don't put in all the little collection and iterator and other structure classes, stick to the "high level" design), and use that to drive your process design. It won't be perfect, but it's not a bad first pass. The profound differences between one execution context and multiple also has subsequent follow-on effects on the languages in question, and in what direction they evolve, so in practice they feel quite different despite the underlying similarities in the model.

Reactive programming is extremely different. There, values are defined as functions on other values, intrinsically, such that if A is B + C, future changes to B and C mean that A is automatically updated to the new B + C with no programmer interaction. This is no longer imperative design; instead of your program consisting of a huge set of instructions for the processor to directly execute (no matter how gussied up that is by the syntax and semantics of the local language), your program consists of a declaration of relationships between values and how they evolve over time. The fundamental operation of the programming language is no longer "Set value identified by X to Y", but "Attach value X to value Y with update function Z".

As is always the case in the world of Turing Completeness, this can be embedded into OO, or OO can be embedded into it, etc. all sorts of combinations are possible. But it is a fundamentally different underlying method for writing your program.

I would observe that A: this isn't new, which I say not because anyone is claiming that it is but because it means that we can in fact draw on past experience to examine it and B: our general experience with it up to this point is that it often makes small tasks easier, even much easier, but that trying to create whole systems this way is often quite problematic. If you think laziness in Haskell is hard to understand, just wait until you're trying to understand a cyclic dependency graph in a data-binding environment (the usual OO manifestation) or a spreadsheet, with the local bizarre adaptations to try to make that sensible all interacting with each other in crazy complicated ways. It's a useful tool, and I don't mean that as a mere rhetorical sop, it really is a useful tool to have in the belt, but I'm deeply skeptical of it as a proclaimed savior of programming, or the Next Big Paradigm. Our experiences with it up to this point have not been positive enough to justify that claim, IMHO, and I haven't seen that there's been any sort of big enough change in the landscape lately to justify that.

Compare to "functional programming", which has been the Next Big Paradigm for a few decades now, but IMHO modern Haskell of the past 3-5 years really is a big enough advance to justify some examination on the topic. (I'm not saying Haskell is guaranteed to be the Next Big Thing, I'm just saying that there's been enough action there to legitimately justify having a fresh look at the situation.)

If you can point me at the big advance for reactive programming in question, I'd love to have a look. I know the Haskell world has been fiddling with FRP but I'm still underwhelmed by the efforts.