There is an entire gamut that runs between 'completely scripted' and 'completed unscripted', with regards to pretty much all entertainment (improvisation included). And even on the 'unscripted' side of the ledger (what we normally consider to be 'improvisation') there are all matter of steps along the line of known and unknown. And everybody here is correct in their various declarations, depending on how down deep they are digging, and how trained the cast is in improv technique.
Is the show 'scripted'? No, it is not. With the exception of the occasional monologue Matt Mercer has written out beforehand (usually campaign openings or very specific bits for specific characters)... no one is reciting lines of dialogue they have created or memorized prior to the show.
Is the show 'pre-planned'? To a certain extent , yes it is, as much as most DMs "pre-plan" their games. To plan for the upcoming session many DMs will write down in their notes several of the likely directions they suspect the players will go, and then create encounters, NPCs, and scene seeds that they can use should the players go that way. If the players go elsewhere? That's fine... the DM can just completely make stuff up as necessary... but more often than not when they know the kinds of leads their players tend to take, they can prepare those directions in advance. Matt Mercer I expect knows pretty well the kinds of threads his players tend to follow, and thus can "pre-plan" things ahead of time.
Do the players have "pre-planned bits"? Yes, and no. In improvisation there are all manner of ways to indeed have 'pre-planned bits' depending on the style of improv and the show type itself. The old Italian Commedia dell'Arte was improvised in that no dialogue was memorized or recited... but every character had comedic bits (called "lazzi") that were standard gags or jokes that they could and would insert into their dialogue and scenes whenever they wanted or needed to. And shows that are running shows (such as Curb Your Enthusiasm or indeed Critical Role) will have character bits that either were created beforehand by the actors and dropped into the shows when they fit... or were jokes/gags that might've been made up on the spot in one particular show, but were then brought back later in follow-up shows. "Lazzi" to use the old term... "running gags" as we call them now. They become part and parcel to who the characters were themselves. And we "normal folks" do this all the time at our D&D tables as well, oftentimes without even realizing it. Any time were create a character and develop their backstory or personality traits, those are the formulations of "pre-planned bits". Maybe not the actual gag itself, but how a gag may manifest is pre-planned. If someone decides to play a greedy character that can't help pickpocketing everyone... that pickpocketing becomes a "lazzi". And it becomes a bit that will be used over and over again.
And finally, there's the question about improv training itself. When you train in improvisation for the stage, you learn all about how to create characters, how to create a scene idea out of nothing, how to acknowledge another player's scene idea and build upon it so that the scene continues, and all the standard "theatrical techniques" that all actor learn about-- presenting out for the audience, diction, voice, timing, staying within a scene, not breaking the 4th wall, not breaking character, reacting to what you receive from your scene partners, etc. Anyone who is theatrically trained (and/or improvisationally trained) knows all of these things and has had them become an intrinsic part of themselves. They are in fact "bits" per se... we all do the "bit" where we just happen to turn three-quarters out to the audience so we can half-see our scene partner and half-present our faces to the audience... but these bits are not
intentionally done, they are just instinctual in us. So other folks may not call them "bits" by the standard everyday sense of what we mean by "bit" nowadays. But both sides could technically claim themselves as correct by stating whether or not that was a bit. And the Critical Role cast have all kinds of these "bits" that they know to do intrinsically for "performing for the camera" to the point where they also don't seem like "bits" as most of us usually use the term. So what they are doing "isn't real" in that they are performing in ways that are meant to help create a filmed presentation, but are also "real" because they aren't making any conscious effort to do them-- they are acting exactly how they would at home. Which for those of us who act or do improv, these techniques are so ingrained that we do them in our everyday lives without even realizing. So we are being "real" and "fake" at the same time.
Critical Role is no different. They're just really, really good.
