If you're with one of America's 4 largest banks (Chase, Bank of America, Citigroup, Wells Fargo), expect a bad time.
I intended to make a more general statement about relatively good banking products, while GP's note only talks about Schwab in particular. I.e., yes, Schwab is a good option, and there are lots of other good options. (Lots of bad ones too.)
I totally agree that a lot of people don't know these things or have good personal finance knowledge in general, and agree that raising awareness of what constitutes a reasonable banking product is a good thing.
They will pull from your brokerage account, if you have insufficient funds they'll take out a margin loan and I think the only fees there are interest.
ING did something similar where they'd only charge you interest for overdraft stuff.
That's not my experience with them. I've had them as a checking/brokerage account for 10+ years. I tend not to keep much money in checking so I run into timing issues once a year or so - where a check I wrote will present before the funds to cover it arrive in my account. I've definitely had them pull from the brokerage cash, but not from savings. They have never opened a margin loan to cover it.
On day four they called me to make sure I was aware. I explained that I had already started a transfer several days earlier, and they said "no problem" and thanked me for being a customer.
No overdraft fees, no pulling from my brokerage account, no margin loan. Never heard anything about it ever again.
Citi's nickname is "shitty." I forget what put me off about Simple but in general debit cards don't enjoy the strong consumer protections that credit cards do in the US.