A professor has invited me for an interview for an internship he is offering. He has asked me to present a paper(that he chose) for 30 minutes. The paper he gave is an object detection paper which is not exactly easy for me.
I am quite nervous for what would be his expectations from an intern.
In general, am I expected to know everything from the paper and be well versed in the field?
It is really hard to tell for us what that specific professor expects. It will be dependent on where you are in your studies, as well as the work you're expected to do during the internship.
Given that they selected the paper, I assume the professor expects it to be something that you should be able to understand. If you're a bachelor's student, you're probably not expected to be able to reproduce or understand every tiny detail of all proofs/algorithms, but you should be able to understand the higher-level structure of the paper. If you're in your master's/PhD, then you should understand more details and also be able to explain certain choices made, or criticize the paper on certain aspects. Regardless of your background, you probably need to be able to position the paper in relation to the prof's research and the internship and talk about that aspect as well.
In general you're expected be able to understand enough of the paper to talk about it for 30 minutes. You say this is not easy for you, but that means that you'll need to spend some more time to understand it. There might be parts that you don't understand. Talking about those parts, being honest that you don't fully get it and discussing how you'd go about understanding it better is also something to discuss during the presentation.
However, I'm speculating here, and the best action is to ask the professor what they expect. That's the only way you'll know for sure.
I had a similar situation when applying for a student position in an ongoing reserach project when I was at university. Although I was not asked to prepare a presentation, I was given a short paper containing previous results from said project in preparation of my interview. Upon handing me that paper, the professor said that they considered this as kind of an intelligence test.
During the interview, they asked me in depth-questions about the topics discussed in that paper, and even questions going beyond the contents of the paper itself, including extrapolating findings from the paper onto other situations. They wanted to test not only my ability to understand a given topic, but also my ability to look beyond, to look deeper into a given objective, in short, they wanted to assess my quality as a researcher.
So even though it might be hard for you, and quite a lot of preparatory work, I would suggest that you do not only use the paper itself for the presentation, but look into one or two related papers as well, and maybe even reference them in your presentation.
In my opinion, it is not necessarily a good idea to ask the professor what they want to hear, because they might expect you - as a prospective person to work with - to "shine" on your own.
