Question: How could Bellatrix possibly have recognised Neville?
Patrick Smith
9th Aug 2019
Harry Potter and the Order of the Phoenix (2007)
Answer: I'd say this is a deliberate movie mistake to provide plot exposition. Bellatrix was one of the Death Eaters who tortured Neville's parents into permanent insanity when Neville was a young boy. It's unlikely Bellatrix would recognize Neville on sight, but this reveals her involvement in the Longbottom family tragedy, and shows that Neville fighting Voldemort and the Death Eaters is personally motivated.
Answer: He looks like his father, wears similar clothes.etc.
Answer: She's seen a picture? Been given his description? There are any number of possibilities.