IN person, William Shatner is quieter and more solemn that you might expect from his performances as Capt. Kirk or from ”Get a Life!” the ebulliently comic, self-deprecating book that he’s in town to promote.

He also doesn’t sound in the least like Kirk did during those frantic moments on the bridge of the Starship Enterprise. Those notorious staccato rhythms, ”I … am … in … command … here,” cherished by stand-up comedians, do not appear in his speech today.

The Canadian-born thespian has never stopped acting (though his best-known role outside the ”Star Trek” series and the subsequent movies was as the superannuated police officer T.J. Hooker). Next week he has a guest appearance as ”The Big Giant Head” in the final episode of ”Third Rock From the Sun.” And he has just come back from the Cannes Film Festival, where he was promoting a film called ”A Free Enterprise.”

”It’s a coming-of-age film that opens in June. I play a character named William Shatner: a total screw-up, vainly attempting to sell a musical version of ”Julius Caesar” in which I play all the parts. It ends with me doing a rap version of ”Friends, Romans, countrymen … ”

He has also tried his hand as a director, and says that he enjoyed directing ”Star Trek V” more than anything else in his long career.

But today he’s really an author, and a remarkably prolific one at that. As well as the current book, which mixes interviews and analysis with some very funny anecdotes, Shatner has written two autobiographies, several ”Star Trek” novels and a whole series of ”TekWar” novels, which were later made into a TV series.

Shatner has no favorites among the successor series to ”Star Trek.” Despite being a sci-fi buff, his television-watching is mostly confined to news and sports. Shatner prefers to spend his time on horseback. ”I’ve competed in world championships on quarter horses and show horses, and I do very well,” he says.

”Get a Life!,” named after Shatner’s notorious ”Saturday Night Live” sketch that skewered Trekkie nerds (and written with Chris Kreski), recounts his attempt to come to grips with the whole Trekkie phenomenon.

In the past, he turned up at ”Star Trek” conventions but would avoid contact with fans, fearful that they might be crazy or stalkers. ”I would breeze into town, do my actor bit and get out,” he says.

Then he decided to find out who they were. As part of that process, he attended conventions in heavy disguise, once being ”outed” by his friend DeForest (Bones) Kelley.

Shatner says that he was struck by the relative ordinariness of Trekkies. ”People focus on the colorful ones. But who really is out there is mom and pop, and Grandma, and the kids.”

Shatner has not yet seen the movie ”Trekkies,” although it and his book are both about the amazing power of the international pop-culture phenomenon that ”Star Trek” has become.

For Shatner, there is nothing wrong with Trekkies. Asked if he thinks it’s a healthy thing for people to worship a 30-year-old TV show, he simply says, ”Everybody needs something. Some people take drink or food or make war. These people are taking a TV show and making it work for them and I think it’s wonderful.”