Find a recording on youtube to get an idea of it, if you don't already.
Don't worry about interpretation so much. No one will probably care.
You follow the soloist. They're right, even when they're wrong. You still follow them. And try to make their mistakes sound correct. If there is anything for interpretation, just go with whatever they do.
They know, or should know, that it takes some time to prepare the piano part. A day probably won't cut it, but you'll great it you do. What you can do -- Cheat. Drop notes, simplify, etc. Nobody cares (depending on what it is). Very few people will notice, even the performer. Who will notice? The professor, but... What do they care? They're focus is on the soloist. Ditto for juries. The judges might notice you tripped up on something, but... What are they going to do? What's anyone going to do?
Everyone knows accompanying others probably isn't your greatest focus.
What it will impact -- Whether you get called back. The soloist and their professors who decide that. I remember one situation where a professor just said, "Don't use x person." That was it. No one in that studio used x person as an accompanist anymore.
Just make the soloist sound good.
The soloist should cue you in. With a breath or nod or both. It should be that obvious. They will probably cut you off on a long note too, although that might be listening more.
Make sure they're positioned so you can see them. Not every soloist knows what they're doing so well.
Before they perform, they should tune. Ask what pitch or chord they want to tune to. They play. You play. They adjust to the piano. It's up to them to tune their own instrument. If it's trumpet, they'll probably want a Bb or C. I heard somewhere that a minor chord makes it easier to tune. I'm not sure how true that is, but I've seen people doing that. And they really already be in tune. The actual onstage tuning is more of a, "Yep, I'm still in tune," situation. They shouldn't, but if they look to you for tuning you can give them a subtle cue (eyebrows up/down). (If they're asking you about tuning... ouch.)
Other than that, just act confident. Polite, etc. On-the-job personality. Thank them. They should thank you too (and pay you, unless you're doing it for a tuition waiver). For walking onstage/offstage, the soloist goes first. Bow when they bow.
Make sure you or the soloist will find you a competent page turner too if you need one.
When in doubt ask, but you could say, "How do you want to handle ____?" instead of saying you haven't accompanied before.
It might be a little odd, but probably not bad if you asked for a recording -- a real one, and one of them, or just record the rehearsal.
Otherwise, for accompanying, you need to be able to play your part with a steady tempo. You need to be able to follow someone cuing you in, pausing you, restarting, etc. Adjusting to the soloist -- If they skip a bar, you have a jump ahead.
Music-wise, you need to be able to play your part and follow their part along. Ideally. It is possible to just have both people start and end together and perform at the same time. Ideally, you've learned their part well enough you can adjust to them better.
Topic: How the heck do you play chamber music?