SLAY TIME

Meet ''Buffy'''s new slay mate. Michelle Trachtenberg reveals her ''Buffy'' obsession -- and how she and Sarah Michelle Gellar go WAY back.

Kids may remember Michelle Trachtenberg as the perky, pint size detective in 1996's ''Harriet the Spy,'' but little ones, take note: Her latest role is less about sleuthing than slaying. The young actress (who turns 15 next month) joins the cast of ''Buffy the Vampire Slayer'' on its Sept. 26 season premiere. As Dawn, a surrogate sister to the Buffster, she gets more than her fair share of teen angst when dark forces use her as a handy human loud speaker for their nasty thoughts. EW.com talked to the 10th grader about her new gig, her history with Sarah Michelle Gellar, and her ideas for a cool Halloween costume.

Q : Were you a ''Slayer'' fan before you got the job?
A : I'm a HUGE fan. I'm a walking encyclopedia of ''Buffy.'' I watched the show from day 1 because I worked with Sarah on ''All My Children'' [Trachtenberg played Lily Benton Montgomery from 1994 to '96, Gellar played mini vixen Kendall Hart from 93 to 95.] I was always thinking of little story lines for myself so I could guest star, like I could avenge Jenny Callendar's [Robia La Morte] death by being her niece or I could be Giles' [Anthony Stewart Head] long lost daughter. I would think of crazy things.

Q : Doug Petrie, who's an executive story editor on ''Buffy,'' also cowrote ''Harriet the Spy.'' Did he put in a good word for you?
A : I didn't even know Doug was working on ''Buffy'' until I went to visit Sarah on the set a couple of weeks before I got the job. I was there, and she said, ''I have a surprise for you. Here's Doug!'' But I found out about the job from my managers and agents, and had a meeting with Joss before I got hired. Sarah was really supportive, though, which was a comforting feeling.

Q : As the only real teenager on a show about teenagers, do you get a little more input than your cast mates?
A : I am the only person under the age of 20 on the set, that's true. Joss always says, ''If we write something for you and you're not comfortable saying a certain thing, great, just say so.'' And we'll talk about certain things and lines from those conversations will pop up in the script later. We all love ''Harry Potter'' on the set, for example. Sarah and Joss introduced me to it. So there was a line about ''Harry Potter'' a little while ago.

Q : Is it tough being the only person on the set who still has to deal with midterms and homework?
A : Not at all. Everyone on the set, especially Sarah, is really fabulous about getting me my time in school and saying, ''Okay, what are you learning now? You have a geometry test to take? Then go, go!'' They won't let me talk to them if there is a possible opportunity for me to run to school between takes. So they're very supportive.