It took four years for the Back to the Future sequels to get underway in the late 1980s and while fans were certainly excited when it finally happened, one of the film's stars was actually quite nervous. Christopher Lloyd recently admitted during a Q&A at a convention that he was "dreading" the return to make Back to the Future Part II because doing so would have meant returning to replay himself from the first film. Lloyd described his major concern...
How do I get back there again, from an acting standpoint.
Christopher Lloyd was joined on stage by his Back to the Future co-stars Michael J. Fox, Lea Thompson, and Tom Wilson over the weekend at Fan Expo Boston (via MassLive) where the foursome spoke about the time they had making the popular trilogy. The first film was a runaway hit which could potentially make anybody nervous trying to recapture the magic that happened the first time around, but Christopher Lloyd says it was something very specific he was expected to recapture that made him nervous.
While the Doc Bown we spend the most time with in Back to the Future was the one from 1955, in Part II we mostly focus on the one from 1985. We see the 1955 version of the character pop in on occasion once our heroes go back in time, but once Doc Brown and Michael J. Fox's Marty McFly get separated Marty must go back to the younger version of Doc Brown for help, where the second film ends with a cliffhanger as we don't know what will happen to Marty.
Trying to recapture a character you played before is probably a potentially difficult problem for a lot of actors when they go back to make sequels, the problem is only compounded when you're dealing with a time travel movie, as you then find yourself playing what is essentially the same character, but one that is still slightly different in many important ways. The Doc Brown of Back to the Future Part II goes through a lot of changes, but then the actor had to go back and find the character who had experienced none of that.
On the plus side, at least for Christopher Lloyd, there would seem to be little reason for him to worry about the possibility of having to find that place again as an actor, as it seems that Back to the Future will very likely remain a trilogy. While Lloyd himself has stated that he'd be willing to return if the right story was ever found for a Back to the Future 4, at this point co-creator Bob Gale has stated such a thing is highly unlikely to ever happen.