The usual procedure in professional US companies is that the company interviews you, issues an offer, and you accept or decline (or negotiate).
This can be finessed/muddied a little bit in the interview process - I've asked candidates "Does this job seem like something you're interested in?" or the like to gauge interest - both as a weed-out (I don't plan to issue an offer to someone who replies "Well, it's a living I guess," I'm looking for a bit more enthusiasm) but also as a data point into how hard to pursue them, the other candidates that might be in play, etc. If they say "totally not" I'll save the work of extending an offer. That's a nonbinding part of the process though, it's not asking you to accept before there's an offer.
I had that happen to me once - a consulting firm flew me out to Chicago for an interview for a six-figure job with an insurance company. Details on what the job were exactly were a little sketchy. I flew out anyway and they insisted I sign an employment contract before the interview and before finding out exactly what the job would entail. I wouldn't budge and they wouldn't budge and I went home. Everyone I consulted with about that later confirmed that was sleazy and unusual on their part and I did the right and prudent thing. Different countries may differ but in the US you should never feel like you need to "accept first" before an offer.
There's also the dance of "salary expectation." Just to be transparent from the hirer point of view, you want to know if this person's going to come in at the amount you usually pay (either by formal salary range or just by general tradition) for that position. Starting points too wide to negotiate waste everyone's time. Though it's OK to rebuff questions on that with a "Whatever you think I'm worth," that's a minor complication not a big "do not hire" brick (to me at least).
In your case, if you are interested, you should get back to them and say "Yes, I am interested in the position and would love to have an offer extended." Unless they make you sign a contract at that point, in which case you should bail, they will then make you an offer and you can decide whether to take it or not.