Felix Leiter is Bond's American counterpart, a CIA agent who is also cool, calm, collected, wears killer shades, and presumably has his own license to kill. He's a good agent, trailing Bond when he worries 007 might be a double agent. And like Bond, he has a sense of humor, teasing Quarrel for drinking when nervous.
LEITER: Hey, Quarrel, if you see a dragon, get in first and breathe on him, you hear? With all that rum in you, he'll die happy!
All that suave attitude rubbed off on actor Jack Lord, who was rumored to want equal billing with Connery and a bigger paycheck to reprise the role of Leiter in Goldfinger. (Source)
He was quickly replaced. Lord went on to star in the original Hawaii Five-O in 1968. Isn't it just like an American to want independence from the British?