In the long-running television series ''Taxi'' he played Rev. Jim, a man transformed by a single joint of marijuana from a straight arrow to a memorably spacey cabby. He's a Klingon villain in ''Star Trek III,'' a prisoner in ''Midnight Express,'' a comically obsessive mental patient in ''The Dream Team,'' the oddball Professor Plum in ''Clue,'' a gambling palooka in ''Eight Men Out,'' an infantile doctor who is spanked (but not kissed) by his mistress in Nicolas Roeg's ''Track 29'' and the evil Judge Doom in Mr. Zemeckis's ''Who Framed Roger Rabbit.''

Considering that gallery, one would have to conclude that Emmett Brown is easily the most accessible and lovable character Mr. Lloyd has played. According to the screenwriter, Mr. Gale, children particularly adore Doc Brown. ''Our sound man brought his 6-year-old daughter to the set one day, and we thought she'd ask to see Michael, but she only wanted to see Chris. Maybe it's his hair.''

Mr. Lloyd, who grew up in Stamford, Conn., says his career path was influenced by an older brother, who is an actor. Also, Mr. Lloyd says, ''I didn't seem to have a knack for anything else.'' When he turned 16 and got his driver's license, he recalls, he ''began going to movies every night. That's all I wanted to do.'' At 19, he moved to Manhattan and began studying with the acting teacher Sanford Meisner at the Neighborhood Playhouse. ''Working with teachers isn't for everyone,'' Mr. Lloyd notes. ''But it really helped me learn what it is I have to do.'' He was Banquo alongside Christopher Walken's Macbeth in a Joseph Papp production at Lincoln Center when Milos Forman chose him to be in ''Cuckoo's Nest.'' ''I looked pretty dramatic at the time,'' Mr. Lloyd says. ''My head was shaved.''

He has barely stopped for breath since, and the praise that fellow actors heap on him is lavish even for actors. ''My relationship with him as an actor parallels the one that Marty has with Doc Brown,'' said Michael J. Fox. ''He's so brilliant, you just enjoy going along for the ride. You never know what Chris is going to do. He could soak himself in kerosene, set himself on fire, and you'd go, 'Hmmm, that's a choice.' If he didn't shock and surprise me in every second take at least, that would shock and surprise me.''

Says Scott Coffey, who co-starred with Mr. Lloyd on another Zemeckis project, an ''Amazing Stories'' episode, ''He's a phenomenally physical actor. He really uses his whole body, which happens to be one of the things Sanford Meisner is known for - teaching actors to be fully present, in the moment and using your whole body.''

Mr. Lloyd, as those who work with him will testify, is a phenomenally shy man when he's not before the cameras. ''I know him less than I know any other actor,'' says Mr. Fox. ''He's very enigmatic.''