To What Degree is Graduate School Acceptance Contingent on You Choosing a Specific Potential Adviser? I applied to a graduate school in CS and got in. However, between application submission and acceptance a professor reached out to me and requested a phone conversation. It went well, as far as I could tell, and then a week after that I got an acceptance. 

The problem is that I listed more theoretical people on my application and not him at all. While he does interesting work, it's decidedly applied work. There's nothing wrong with that, I'm still interested in the applied person, but I'm not sure if my acceptance offer (which comes with partial funding) is contingent on me working with this person. 

How does this generally work? Am I stuck with this person because I didn't reach out to the other people? To what degree can I still 'pick' my advisor? Thanks!
If you are accepted, then you are accepted. The prof is trying to "capture" you early, which is fine, but you don't have to agree to it. In the US you can generally choose a research advisor fairly late, especially if you come in with only a BA/BS degree. 

There should be no problem at all if you don't want to work with them. But, it would be worth projecting a friendly and flexible face until you see more of the new program and get to meet a few people and (probably) take a few classes. 

Your first goal will be to take enough advanced classes that you are successful in passing comprehensive exams. Then, you get more serious about research topics, and, for many, even research areas. 

Note that in some other places, professors (PIs) actually hired their own graduate students. This is less common in the US and hardly ever in CS here. In the US, it is a department, not an individual, that accepts you, generally with the advice of a committee of faculty. 

But if anything were contingent on anything else, then it would be explicitly spelled out. The prof is "reaching out" because you seem "interesting." Nothing more than that.