A series of promotional videos for Rick and Morty's season five finale brought the animated characters into live-action. Actors Christopher Lloyd and Jaeden Martell portrayed the "real-life" counterparts of Rick and Morty, respectively.
Lloyd previously appeared in the Back to the Future series, which Rick and Morty originated as a parody of. Morty gets his name from the series' teenage protagonist Marty McFly. And Lloyd's character, the time-traveling mad scientist Emmett "Doc" Brown, was the main inspiration for Rick. "Christopher Lloyd" trended on Twitter after Adult Swim released the first clip of him as Rick, with many fans describing him as the ideal casting for the character.
This crossover has been a long time coming: back in 2019, GeekTyrant posted an interview with Lloyd where he expressed his love for Rick and Morty. He suggested that the show's characters could appear in a potential Back to the Future 4, running into Doc and Marty "in another time zone, in another space zone." But since another Back to the Future sequel isn't happening according to franchise co-creator Bob Gale, bringing "Doc" into Rick and Morty might be the best alternative.
The captions of the two clips seem to indicate this is a separate Rick from the animated ones fans are most familiar with. The first is marked "C-132", while the second is marked "C-1.21". These refer to the separate realities that exist in the Rick and Morty series (the show's characters are from Dimension C-137). This likely means that, canonically, there are several live-action versions of the characters existing in alternate universes.
The first clip shows the two characters stepping through a portal into Rick's workshop, with Rick exclaiming "Morty... we're home!" This shows that like their counterparts in C-137, the duo in C-132 also goes on inter-dimensional adventures. More interestingly, however, the second clip calls back to the infamous "Pickle Rick" episode from season three, in which Rick turns himself into a pickle. Morty sees a pickle and exclaims "Rick, you did it again! Didn't you learn anything from last time?", meaning that some variation of the events of that episode also happened in C-1.21.
Of course, it's also possible that these universes are simply part of an Adult Swim marketing ploy, and that they have no basis within the actual series. When SYFY WIRE asked Adult Swim about this, they merely gave the cryptic statement "you never know what surprises are in store." But given how big of a fan Lloyd is of Rick and Morty, it seems like he'd want to make an appearance in the series itself, not just as part of a promotional campaign.
Source: Adult Swim|Twitter