How does cyanide and other substances kill so quickly?

Now that I think of it, the instantaneous death of spy movie fame isn’t really so dramatic as the Shakespearean poisons that permit characters to give a stirring final speech before their death, but are not so slow-acting as to make them suffer the indignity of dying off-stage. =)

How about the indignity of puking their guts out?

Romeo: O true apothecary! (vomits on Juliet’s bodice) Thy drugs are (vomits some more) quick. Thus with a (vomits bile) kiss (dry heaves) I die. (lands face first in his puke)

Loses some of the romance, I suppose.