The most moving speech of the American Film Institute's tribute to Jimmy Stewart came not from a contemporary of Stewart's but from actor Dustin Hoffman. "It's a mistake that I'm up here tonight," he told the 1,200 guests at the Life Achievement Award dinner at the Beverly Hilton Hotel. "It's very complicated . . .

"I just wanted to be here, and I wanted my parents to be here. My father worked with you on the lot when you wer e making films with (director) Frank Capra. He's your age. My mother is my age."

After noting that he'd just seen Stewart's favorite film, the Capra-directed "It's a Wonderful Life," Hoffman concluded by saying: "You made me laugh, you made me cry, you made me wish for a country we perhaps haven't seen for a while. You made my parents very happy, you made me very happy, I'm going to see to it that you make my children very happy, and if this world has any kind of Capra luck, you're going to make my children's children very happy."

And so it went for James Stewart, an actor whose greatest gift is his ability to be so unusually usual. He was lifted above the commonplace Thursday night when the AFI presented him its eighth Achievement Award and Stewart noted, with typically hesitant, drawling modesty, "I guess you could say that up to tonight they have honored brilliance, daring, and abundance of talent, and that brings us down to where we are. Now the American Film Institute in all of its wisdom has added a new name and a new category; James Stewart, a remarkably fortunate fellow."

The guests, who dined on authentic Royal Stewart tartan tablecloths, included old-time directors like King Vidor, Mervyn LeRoy and Henry Hathaway, as well as the expected cross-section of fellow actors ranging from Ernest Borgnine and Fred MacMurray to Michael Caine and Nick Nolte. Overshadowing them all, however, and just about overshadowing the guest of honor himself, was a certain former actress now living abroad.

It was H.S.H. (Her Serene Highness you understand) Princess Grace of Monaco, looking genuinely serene in a puffy-sleeved gown of royal purple, who attracted by far the greatest share of photographers as well as the envious attention of just about everyone in the room. "I think she looks as beautiful as ever," said one woman while her neighbor, a critical type, asked, "Isn't her hair a shade lighter than it used to be?"

Master of ceremonies for the evening was Henry Fonda, the life-achievement winner two years ago and a friend of Stewart's for nearly half a century -- going back, he said, to the time when both had shared a "fleabitten apartment within walking distance of Times Square costing $7 a week."

"I'd worn out my shoe leather for five years, but Stewart never went out and looked for a job, parts just kept happening for him. I'd go back to his dressing room and look at him and wonder when and where and how in the hell did he get to be so good."

If there was a secondary honoree at the dinner, it was Frank Capra, a teeny-weeny man who directed Stewart in "Mr. Deeds Goes to Town" and "Mr. Smith Goes to Washington." He received the only standing ovation of the night aside from Stewart's, and told the audience, "I'm going to bore you with some stuff I've written."

Capra touched on the aspect of Stewart's acting that is perhaps most appealing: his naturalness. "There is a higher level than great performance, he said. "There is a lever of no acting at all when the actor disappears and a real live person appears on the screen, a person the audience cares about immediately. Very few people are on that level, and that tall stringbean over there, he's one of them."

Other speakers on the program, which is scheduled to be broadcast on CBS on March 16 at 9:30 p.m., included Beulah Bondi, five times Stewart's film mother and suprisingly spry at 87, and Walter Matthau, who served under Stewart in the 453rd Air Force bombardment group during World War II and who cracked at the cue card, "I'm glad you have 'two' written out instead of the Roman numbers. I think it was Lawrence Welk who once said, "World War Eye.'"

Finally, Stewart, still trim and lean at age 71, had his turn. "This has been a wonderful evening that is about to go downhill as I fumble for the right words," he began. "I know it's late, and I promised myself I'd talk fast. The problem is, I don't know how to talk fast."

The actor then introduced his family, ending with his wife -- "there's my Gloria, bless her heart" -- and then thanked the people who had helped him, starting not with producers or directors but with makeup man Frank Westmore, and his double and stand in, Ted Mapes. He thanked Dustin Hoffman -- "My God, that was wonderful" and then he tried to sum up how it all made hm feel.

"It's kinda like tying a happy ribbon around a wonderful lifetime of getting paid for what I love to do," he said. "I'm just grateful to all of you for the opportunity you gave me. I'm grateful for every wonderful day in this wonderful business, and I thank you."