As I’m finishing my supplements, I’ve started to wonder how colloquial my responses should be. I don’t want to sound stuffy or like a thesaurus, but would a school think I’m not “intelligent/mature” enough if I had a more relaxed feel on some of my responses? Obviously I’m not going to use bad words or slang–I’m talking things like “geeking out”, “(read: weird)”, etc. And what about exclamation points? Should I avoid them altogether? I’ve always heard “we want to hear your voice”, and I’m trying to channel that. I read some of the Tufts sample essays that worked and they sounded really down-to-earth, so I think that’s good to emulate, but for other top schools (like Yale) should I err on sounding more put together?
Conversational is good and the examples you give are fine. A good rule of thumb: If you write “however,” “therefore,” or “thus,” you’re doing it wrong. @corbeaux
@marvin100 thank you!
You’re welcome–best of luck, @corbeaux !
I would like to now see a student challenge your statement @marvin100 . I am imagining someone out there could come up with a very clever essay full of big, important words:-) However, I do agree that you are correct. Thus, I encourage OP to be him or herself and write naturally, but PLEASE avoid text-speak. I have read a few essays lately in which students are unnecessarily abbreviating words (probs, for probably, as an example), and therefore, I urge the OP to use discretion when being conversational. 
Seriously though, there is a lovely student out there, @pocrocodro , who knows how diffcult it is to sound like a 17 year old, because his interfering parent has edited his essays (against the wishes of procrocodro) to the point where they sound like entries in an encyclopedia. This is to be avoided. I think it’s fine to say geeking out, and I would get that and enjoy reading it. I also think there is no harm is using a few well-placed “big” words every now and then, if it enhances the essay and blends with the overall tone. The most important thing is to try and reflect who you are. You want the reader to think “I like this student and I want him/her at this college.”
Haha totally–it’s definitely doable, and there are exceptions to every rule (especially “rules of thumb” 
), for sure. I had a student once who got into Yale and dropped two F-bombs in their Personal Statement (!), so you never know!