At interview she was casually dressed with good hygiene and engaged well.
At the interview she was casually dressed with good hygiene and engaged well.
It is not necessary in this specific context to use the definite article because, from the point of view of the writer, the sentence NOT is about some specific situation about some specific interview. It is about the interview stage in some organisation's recruitment process.
The hygienic lady who was interviewed would undoubtedly refer to 'the interview' but for the writer, apparently the person reporting the results of that interview, who has probably interviewed dozens of applicants, it is a generic description of that part of the process.
The usage is exactly the same as you will find in the medical profession: 'the patient presented with a painful shoulder, on examination a growth was found, in surgery more complications emerged, in convalescence the symptoms abated'.
From the context, it is evident that the sentence is about some specific situation about some specific interview. "Interview" is not an abstract term in this case, so it is necessary to use the definite article. Please view The definite article
Use the to refer to something which has already been mentioned.
Source: (https://www.ef.com/wwen/english-resources/english-grammar/definite-article/) - EF Education First English Grammar Quide
Interview is a countable noun with the plural form being "interviews" (there's no abstract form that I know of).
An article is definitely needed or it would be grammatically incorrect.
If the listener doesn't know what particular interview you're talking about use At an interview and if the listener is aware of the context use At the interview
Hope that helps.