Passive vs. Aggressive Order Placement
Every order is one of two kinds, and the difference is who pays the spread.
Passive orders rest in the book and wait. You post a bid below the market and someone comes to you. You earn the spread, and on most venues you also earn a rebate for providing liquidity. The cost is that you may never fill.
Aggressive orders cross the spread and execute now. You buy at the offer. You pay the spread plus a taker fee, and you get certainty.
The hidden cost of passive orders
Non-execution is not the only risk, and it is not the worst one.
The subtler problem is which passive orders fill. Your resting bid fills when sellers are eager, which is disproportionately when the price is about to fall. You get filled on the trades you would rather not have, and miss the ones you wanted. This is adverse selection again, and it means a passive strategy's realised P&L is systematically worse than a naive spread-capture calculation suggests.
There is also opportunity cost: while your bid sits unfilled and the market runs away from you, you miss the move entirely.
A passive order does not fill randomly. It fills when the other side is motivated, which correlates with the price moving against you. Always subtract expected adverse selection from expected spread capture.
When to cross the spread
A market maker is passive by default, since the spread is the revenue. But there are clear cases where paying it is correct:
Inventory is beyond limit. Waiting for a passive fill risks the position moving further against you. Paying a two-cent spread to eliminate a position that could lose fifty cents is obviously right.
Your fair value moved and you need to reposition. If you know the price is now higher, buying at the old offer is buying below your new fair value. That is not paying the spread, it is taking value.
You are hedging. A hedge that arrives late is not a hedge. Execution certainty is usually worth more than the spread.
Approaching a known event. Getting flat before an announcement is worth paying for, since spreads after the release will be far wider than the cost of crossing now.
The cost of hesitation
The comparison is not "spread now versus zero later". It is "spread now versus spread later plus whatever the price does in between". In a fast market the second term dominates the first quickly, which is why experienced traders cross decisively rather than repeatedly re-posting passive orders slightly behind a moving market.
Chasing a moving market with passive orders is a recognisable failure mode: you never fill, the market keeps moving, and you end up crossing anyway at a much worse price.
If you have repriced a passive order more than twice chasing the market, cross. The accumulated slippage from chasing usually exceeds the spread you were trying to save.
Mixed approaches
Most real execution blends the two. A large order might rest passively for the bulk of the size and cross for the remainder as a deadline approaches, which is roughly what execution algorithms do. See TWAP and VWAP.
The blend is governed by urgency: the more time you have, the more passive you can afford to be.