A few items to note (all based on IB)
1) They can't loan out your shares unless you authorize it, hence the "stock yield enhancement" program (which you have to opt in to...it is not active by default:
https://ibkr.info/node/1838. Note that you can't select piecemeal. You either allow them to borrow any shares in the account or none. You probably want to read the specifics of the program where they go into the details on the process. Note: 1) You can still sell calls against the stock, 2) you can sell your stock at any time, 3) you can opt out of the program at any time.
2) Nothing guarantees they loan your shares out. If they do, you split the rate 50/50 (can't recall the link, but dig on their site or ask customer service). Currently on NKLA, it's 897%. You can check this by adding a "shortable" column on TWS. It'll be lime green for easy to borrow and dark green for hard to borrow and will show the number of shares available for shorting.
3) Interest is calculated based on the closing price. Interest is an annual figure, so the daily rate is approximately %/100 * Closing Value/365. There's also a bit of rounding done:
https://ibkr.info/article/1146 so it won't exactly match the above calc.
4) If you are assigned and don't have shares, you automatically are going to incur 1 day of interest. You won't know if you have been assigned until about 10pm PST (which is when I see assignments/expiration post in IB). I seem to recall IB saying they post it after their daily system update which is normally around that time, but I can't find that link. You can easily confirm it with an IB chat with customer service. If you are assigned on Friday, you're obviously on the hook for two days.
5) Basically you have the extended session (5pm PST) to pick up shares to cover your assignment. After that, you have to wait for pre-market, which is 1am PST the following day, so it won't matter if you cover then. If there is not a lot of time decay left in your shares, there is a pretty good chance you can be assigned, especially if it's getting near expiration. There's always a chance that somebody who knows (or think they know) something which may make them want your shares and be willing to eat the premium.
6) The fee won't hit until settlement (T+3), so it takes a few days to post in your account. You'll see it in the "MTD interest"
If I missed anything or posted any errors, please let me know (especially if def from IB wants to chime in).
