It is meant to be a bit painful in all circumstances. It isn't a pleasant way to die, but it doesn't look all that bad to watch, which is why it is used. Pain and trauma free measures like nitrogen asphyxiation aren't used for that reason; pro-death penalty advocates want to keep it painful.

Of course it is, that's why they like to use it. It's the perfect crime really, paralyze the guy so he can't protest then dump salt into his heart.

If they wanted it to be painless, they'd just throw in more of the barbiturate, which they're using already anyway. There is a reason we kill animals like that. Or, as another of your respondents pointed out, nitrogen asphyxiation. That one is so impossible to mess up it's absurd.

Occasionally it is. I remember there have been instances where the sedative in the cocktail didn't work, but the paralyzing and killing agent did. Wasn't obvious from the outside, but it must have been a pretty awful death...