A quant researcher usually spends more of the day asking whether an idea is supported by the evidence. A quant trader spends more of it deciding how the desk should trade given the evidence it has and the market it faces. On a small automated desk, the same person may do both. The split is easiest to see when something goes wrong.
Say the desk has been quoting a product and the trades it gets in the first half-hour are worse than expected. The trader has decisions to make while orders are live. Are the quotes stale? Has the market moved faster than the system can respond? Should the desk reduce size, widen the quote, or pause while it checks the data? If a trade goes through, someone still owns the exposure. The trader is closer to that responsibility and to the systems that control it.
The researcher can take a longer look at the same event. Were these fills unusually bad relative to other days with the same volatility? Did the fill model miss a pattern around announcements, or was there a timestamp problem in how the fills were matched to prices? Much of the work could be reconstructing what was known before each order went out, agreeing on a baseline, and running the test on days the desk did not use to form the hypothesis. A good outcome might be a change to the quoting model. It might also be a short explanation of why the apparent pattern disappears with better data.
On a systematic fund that trades daily or weekly, the trader's day can look quite different. They might work through the orders a portfolio process has produced, compare actual execution with estimated costs, and investigate why an instrument is hard to trade. The researcher may spend days on a single data problem or own the code that turns forecasts into portfolio weights. On an options market-making desk, traders may be much closer to pricing, hedging and inventory decisions, and researchers may work on the models those decisions depend on. Programming can be central to either role.
If you're comparing roles, ask what the person in each seat owns. Who investigates a bad month? Who decides whether a model change goes live? Who can change a limit or pause a strategy? Those answers tell you far more about the job than whether the title says “researcher” or “trader.”
For a feel for the trader's side of that decision, try quoting a two-sided market and watch how fills change your inventory.