White House press conferences are often pallid and pre-scripted affairs. But President Obama’s occasionally testy exchange with reporters this week revealed a key dimension of his approach to the presidency.

The question was Iran, and the charges leveled by Republican critics were that he’d been too “timid and passive” in denouncing the repression of political dissent. After acknowledging the “genuine passion” of those critics, Obama asserted: “But only I’m the president of the United States. And I’ve got responsibilities in making certain that we are continually advancing our national security interests.”

In other words, it’s easy to appear on television or speak in the Senate and toss out popular applause lines about the evils of tyranny. It’s much harder to sit in the Oval Office and calibrate your language, knowing that every word you say will be distributed and dissected around the world.

When you have “responsibilities,” the president was saying, you can’t afford to see the world in black and white – or even bright green, the color of the Iranian protestors. And you can’t indulge your emotions and reach for the quick headline or sound bite. “I know everybody here is on a 24-hour news cycle,” he snapped at reporters who pressed him for sharper words and clearer threats. “I’m not. OK?”