Quant processes are lighter on behavioural questions than banking, but they are not absent, and a bad answer to a small number of them does real damage.
"Why trading?" / "Why this firm?"
Asked almost universally, and the wrong answers are well known: money, prestige, or a generic interest in markets.
What works is specific and honest. An interest in decision-making under uncertainty. Enjoyment of fast feedback loops - you find out whether you were right in hours rather than years. A genuine enthusiasm for games with incomplete information. These are true of people who last in the job.
For "why this firm", something concrete about the process, the desk, or something the firm has published beats anything from the front page of its website. The company interview-process pages are a reasonable starting point for finding something real to say.
Resilience and competitiveness
Firms probe these deliberately because the job supplies frequent, unambiguous evidence that you were wrong. Expect questions about a time something went badly, or about a competitive setting.
Answer with something you actually lost. A story where you competed and won is much weaker than one where you competed, lost, worked out why, and came back - because the second demonstrates the thing being tested.
"Tell me about a time you were wrong"
The central behavioural question of this industry, in whatever form it arrives.
A strong answer has: a real error, a clear account of why you made it rather than bad luck, what you changed, and no defensiveness. A weak answer is a disguised strength or an error caused entirely by someone else.
Specific beats impressive
A small, true, well-remembered story outperforms a large vague one every time. Interviewers ask follow-up questions, and detail is the thing that survives them.
What they are not testing
Polish. Nobody in this industry is scoring your delivery. Being obviously rehearsed is worse than being slightly awkward and clearly honest.