Versatility can cause some actors to be sentenced to an abundance of characters roles.

Not so with Derek Jacobi.

From his first triumph on the American scene in the PBS series as the stuttering but resourceful Roman emperor in "I, Claudius," to his most recent TV outing as the maniacal Adolf Hitler in the recently televised "Inside the Third Reich," Jacobi ends up playing a wide variety of leading roles.

He is currently "starring" in "The Secret of NIMH," an animated action fantasy in the classical style, set to be released in July. The film, an Aurora presentation, is the first feature from Don Bluth Productions. MGM/United Artists Entertainment Co. will release the film in the U.S. and Canada.

Jacobi gives voice in it to Nicodemus, the esteemed and venerable leader of a group of mysterious and intelligent rats who are summoned to help a widowed field mouse trying to save her family. He lends his vocal talents with Elizabeth Hartman, Dom DeLuise, Peter Strauss, John Carradine, Hermione Baddeley, Arthur Malet and Paul Shenar. During the recording sessions, which occurred early in the production of the film, Jacobi found and settled into an interpretation of what he felt Nicodemus' voice should be.

"The only problem I had," the actor confesses good-naturedly, "was to keep sounding old enough. It's a wonderful problem, really."

Jacobi, long recognized as one of England's most gifted classical actors, is one of the eight people chosen by Sir Laurence Olivier as founding members of Great Britain's National Theatre Company. He won the 1976 best actor award from the British Academy of Film and Television awards for his portrayal of Claudius.

More recent credits include leads in PBS productions of Shakespearean plays including "Hamlet" and "Richard III" and films "The Human Factor" and "The Enigma." He made his Broadway debut in 1980 in the Russian satirical comedy, "The Suicide," which was produced by the StageWing of Aurora. He also starred as Burgess in the television docu-drama, "Philby, Burgess and MacLean."

Jacobi lives in London and commutes to America when he is working here.

"The Secret of NIMH" was produced by Don Bluth, Gary Goldman and John Pomeroy and directed by Bluth. Jerry Goldsmith composed and conducted the songs and score, and Paul Williams wrote the lyrics. The story was adapted by Bluth, Pomeroy, Will Finn and Goldman from the Newbery Award-winning novel, "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien. Rich Irvine and James L. Stewart are executive producers.