Before expiry
Time decay, or time erosion, happens continuously for an option and becomes more pronounced as expiration approaches. Time decay is favourable for you when you’ve sold (shorted) a call to open. This means that, all else being equal, the value of a call option declines as time passes and, as a result, your liability as the call seller also declines. This happens even if the stock price doesn’t move.
A drop in the stock price is another positive for the option side of the covered call trade but you’ll also experience a drop in the price of your stock holding. This is why a covered call provides some limited downside protection for the holder of the underlying stock. If the stock price drops dramatically, the extra premium gained from selling the call option might not be enough to offset the loss.
If the stock price moves higher, the call’s price is likely to increase. Because you’re short the call, your liability on the short call increases as the stock price moves up. You’ve been paid the call’s premium in exchange for taking on the obligation to sell your stock at the call’s strike price on or before expiry. Once the market price of the stock is higher than the call’s strike price, your obligation means you are no longer benefiting from the stock’s appreciation. You can buy back the call, closing the short call position and removing your obligation before expiry, a trade called a “buy to close”, but you may have to pay more to do that than you initially took in when you shorted it.
A drop in implied volatility will generally cause a call option to lose value, a positive for you as the option seller if you’ve already shorted the call — it will be cheaper to buy back if you decide to close out the position before expiry. Conversely, a spike in implied volatility will almost certainly push the option price higher, a negative for option sellers who are already short calls. Implied volatility can increase before an earnings release, for example, then drop substantially if the announcement contains no major surprises and the stock price has little reaction. Implied volatility could also increase after the announcement if the stock reacts with more volatility than previously expected.