Some of the Tum.

American English loves the schwa. Unless we start to look very closely at the phonetics of pronunciation we can transcribe the phoneme as the standard lax central vowel [ə]. Several vowels in AmE can alternate with this sound when in an unstressed position. Consider the following pairs of stressed (or emphatic) and unstressed pronunciations of words/syllables.

the: [ði] - [ðə]
forever: [fo.rɛ.vər] - [fə.rɛ.vər] (Final [r] is probably syllabic--the first may be as well.)
mistake: [mɪstek] - [məstek] (is the [s] possibly syllabic?)
that: [ðæt] - [ðət]
to: [tu] - [tə]
A: [ej] - [ə]

and so on...

When a vowel that is almost always unstressed gains a necessary (or artificial) stress the new vowel varies dialectally. I've heard some Canadians stress the indefinite article by saying [æː]. In the U.S. it's almost always stressed [ej]. When I was in the Glee Club at the University of Michigan we learned to sing the last syllable of "Michigan" as if it rhymed with "gone." A few listeners made a point of telling us that we were pronouncing it incorrectly. "You're supposed to say it 'Michi-gen'" they offered. They obviously were interpreting underlying form of the schwa as the [ɛ] in red bed get them... UM Glee Club phonology says that when stressed its the [a] of father hot otter bomb. Okay it's not actual phonology at all. It's more of a this-vowel-is-the-best-for-singing system.

So what does this have to do with Buffy? When a vowel is stressed she turns it into the stressed open-mid vowel symbolised by the "carrot" or "hat" [ʌ] that we hear in cut love sun... (technically the open-mid back unrounded vowel). I've heard other people do this but not as clearly as she does. The two words in which she does this most clearly are 'to' and 'because.' Our Austrian friend Norbert cracks up whenever he hears her say this. What he I find so funny is the stress she puts on these segments. When she says "because" [bʌkʌːz] as a clear spondee the two syllables have the same vowel. And when she says 'to' [tʌː] she often lengthens it dramatically while she's making her point--either gesticulating or thinking of a good word: "You know how little kids like tuhhh..." (her hands flailing) "frolic and flounce crazily..."

This is a drastic neutralization of two vowel sounds that are underlyingly quite far from their usual pronunciation. When unstressed they're almost identical [ə], while most people will stress them as high front tense [i] and high back tense [u]. Buffy's surface form for emphasis puts them both in the same place [ʌ]. This shouldn't strike us as too odd considering that most Americans do this also--and with the same word. Sometimes 'the' is stressed with the high front [i]--"are you the Donny Most?" and other times as the open-mid back unrounded vowel [ʌ]--"Wouldn't you know it? The one time I choose to miss class..."

[The last stressed 'the' is harder to illustrate contextually. It's not a standard with a clear semantic meaning like the celebrity indicative 'the'.]