On the big screen, Murphy, who turns 25 next week, has built a career on similar feats of alchemy. Since her debut in 1995’s “Clueless”–she had curly brown hair then and puffy cheeks, perfect for playing Tai, the transplanted Jersey girl–and up through last year’s thriller “Don’t Say a Word,” the actress has specialized in taking one-note parts and shaping them into memorable creations. She does it again in “8 Mile,” which opens Nov. 8 and features the film debut of Eminem, a.k.a. Marshall Mathers. The movie is a sort-of-autobiographical story of a Detroit kid whose microphone skills could be his ticket out of a trailer-park life. Murphy’s one-note role this time? The girlfriend. So far, all of the “8 Mile” chatter has been about Eminem–can he act or not? (He can.) But Murphy is the one you’ll remember.

She plays Alex, a gifted woman who, like Eminem’s character, Jimmy Smith, is desperate to escape the dumps of Detroit. (“8 Mile” is a reference to the famed city street that divides the black and white sides of town.) Alex’s gift, though, is her face. She wants to be a model, but where she’s from, it takes more than giant eyes and glowstick skin to become one. The film is about seizing your opportunities, which is exactly what Murphy did in taking the part. She was offered the role just after September 11, and she drove through the night to Detroit to arrive in time for rehearsals. She listened to Eminem CDs the whole way. “I was nervous–I was about to meet this force of nature. But we had all just gone through something huge, so I wasn’t gooey star-struck,” she says. “I saw the bigger picture.”

When they finally met, Murphy, rather uncharacterically, did not hug the force of nature. They shook hands. “A firm handshake, which is extremely important,” she says. The relationship evidently escalated in a hurry. During filming, the gossip mill hummed with stories that Murphy and the rapper’s on-camera love scene was, shall we say, well rehearsed. Both parties took turns issuing flirty “no comments.” Eminem teased reporters, saying, “I’ll never tell”–a homage to Murphy’s secret-keeping psych patient in “Don’t Say a Word.” The actress kept inquisitors at bay by dropping an Eminem lyric: “I am whatever you say I am.” She still demurs. “I want to respect the one ounce of privacy he has left and the one ounce that I’m losing,” she says. “I’m just grateful to have had him, in any way, shape or form, in my life.” Yes, “had”–past tense. Murphy, it seems, has pulled off the impossible: an amicable breakup with Eminem.

The actress was born and raised in New Jersey by her single mom; her parents split up when she was 2. Murphy’s first big role was in a regional theater production of “Really Rosie.” She was 9. “The local news station interviewed me,” she recalls, “and the reporter asked me, ‘So what are your plans?’ I said, ‘Well, first I’d like to go out to the coast and work in TV, then I’d like to come back and do Broadway and then I’m gonna go back out to the coast’–I kept saying ’the coast’–‘and be in films. Then I’m going to be a singer’.” So far, so good. Murphy’s already done Broadway–Arthur Miller’s “A View From the Bridge” in 1997. On Nov. 16, she’ll fulfill a lifelong dream and host “Saturday Night Live.” (“Holy moly, I’m so excited. This, to me, is cooler than anything ever! It’s the end-all, be-all! I mean, wow.”) Next year, she’ll get top billing in a pair of films: a comedy with Ashton Kutcher called “Just Married” and the title role in “Molly Gunn,” about a trust-fund brat who loses all her dough.

On every subject besides Eminem (whom she calls Marshall), Murphy is an eager, mile-a-minute talker. She likes words that end with the letter Y, like “yummy” and “belly” and “bunny.” And she laughs constantly. It’s a terrific laugh because it’s so not pretty. It is deep and hiccupping and very ungirlish. Any time you make her laugh you know it’s genuine, because no one would laugh this way on purpose. Early in the interview, Murphy grabs the tape recorder off the table and, unprompted, sings a few bars of a slinky jazz standard. “Siiiilent days, shady niiights.” What’s the tune? “Oh, it wasn’t anything,” she says. Then the laugh comes. “It was just me futzing around.” Talk about a talented girl. All this, and she can futz, too.